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HEMICAL        HILOSOPHY. 


BY 

JOHN    HOWARD    APPLETON,   A.M., 

Professor  of  Chemistry  in  Brown  University, 
AUTHOR  OF 

"  BEGINNER'S  HANDBOOK  OF  CHEMISTRY,"  "  THE  YOUNG  CHEMIST," 
"  QUALITATIVE  CHEMICAL  ANALYSIS,"  "  QUANTI- 
TATIVE CHEMICAL  ANALYSIS." 


SECOND 'EDiTiQN. 


SILVER,  BURDETT  &  CO.,  PUBLISHERS, 

NEW  YORK  .  .  .  BOSTON  .  .  .  CHICAGO. 

1890. 


PROFESSOR  APPLETON'S  WORKS  ON  CHEMISTRY. 


I.  THE    BEGINNER'S    HANDBOOK    OF    CHEMISTRY:     Price  $1.00. 
This  is  an  introduction  to  the  study  of  Chemistry,  suitable  for  general  readers.     It  treats 
chiefly  the  non-metals,  these  being  generally  found  to  furnish  the  best  material  for  an  ele- 
mentary course,  and  to  best  illustrate  the  fundamental  facts  and  principles  of  the  science. 

The  book  is  written  in  attractive  style,  and  has  had  a  very  large  sale.  It  is  profusely 
illustrated  with  engravings,  and  has,  in  addition,  fourteen  colored  plates. 

II.  THE  YOUNG  CHEMIST:    Price  75  Cents.      A  book  of  chemical  experi- 
ments for  beginners  in  Chemistry.     This  is  designed  for  use  in  schools  and  colleges.     It 
is  composed  almost  -entirely  of  experiments,  those  being  chosen  that  may  be  performed 
with  very  simple  apparatus.      The  book  is  arranged  in  a  clear,  systematic,  and  instructive 
manner. 

III.  QUALITATIVE  ANALYSIS  :  Price  75  Cents.    A  briei  but  thorough  manual 
for  laboratory  use. 

It  gives  full  explanations,  and  many  chemical  equations.  The  processes  of  analysis 
are  clearly  stated,  and  the  whole  subject  is  handled  in  a  manner  that  has  been  highly 
commended  by  a  multitude  of  successful  teachers  of  this  branch. 

IV.  QUANTITATIVE   ANALYSIS:   Price  $1.25.     A  text-book  for  school  and 
college  laboratories. 

The  treatment  of  the  subject  is  such  that  the  pupil  gains  an  acquaintance  with  the  best 
methods  of* determining  all  the  principal  elements,  as  well  as  with  the  most  important 
type-processes,  both  of  gravimetric  and  volumetric  analysis. 

THE  EXPLANATIONS  ARE  DIRECT  AND  CLEAR,  so  that  the  pupil  is  enabled  to  work  intelli- 
gently even  'without  the  constant  guidance  of  the  teacher.  By  this  means  the  book  is 
adapted  for  self-instruction  of  teachers  and  others  who  require  this  kind  of  help  to  enable 
them  to  advance  beyond  their  present  attainments. 

V.  CHEMICAL   PHILOSOPHY  :     Price  $1.40.      A  text-book  for  schools  and 
colleges. 

It  deals  with  certain  general  principles  of  chemical  science,  such  as  the  constitution  ot 
matter;  atoms,  molecules,  and  masses;  the  three  states  of  matter  and  radiant  matter; 
the  change  of  state  from  one  form  of  matter  to  another.  It  also  presents  such  topics  as 
Boyle's  and  Mariotte's  law,  Charles's  law,  and  the  other  general  laws  of  matter.  It  dis- 
cusses from  a  chemical  standpoint  certain  forms  of  energy,  such  as  heat,  light,  electricity. 
It  treats  of  the  nature  of  chemical  affinity;  the  chemical  work  of  micro-organisms;  the 
modes  of  chemical  action;  thermochemistry;  and  those  attractions  of  substances  which 
are  partly  physical  afld«pajtty  chemical.*  Jt^lsp  Jjre£ents  a  lull  study  of  atomic  weights, 
the  methods  leading  to  a  5rs»f  adopticvn/oirtKeni",  §nd;then  to  the  grounds  sustaining  cer- 
tain numbers  selected.  Trie  periocTic  system  is 'of  course  discussed. 

The  work  is  fully  iilt*str?.te,d.'  r  ' ,  "  ; 

Copies  senf 'by  ih'aii, ''postpaid;  by r tire' Publishers,  upon  receipt  of  the 
advertised  price. 


COPYRIGHT,  1890, 
BY  JOHN  HOWARD   APPLETON. 

TYPOGRAPHY  BY  J.  S.  CUSHING  &  Co.,  BOSTON. 
PRESSWORK  BY  BERWICK  &  SMITH,  BOSTON. 


PREFACE. 


THIS  book  is  a  formal  presentation  of  certain  subjects 
which  the  author  has  been  in  the  habit  of  offering  to  his 
classes  in  the  form  of  lectures.  It  is  intended  to  explain 
to  beginners,  or  even  tolerably  advanced  students  in  chem- 
istry, certain  of  the  general  laws  of  the  science,  and  that  in 
a  compact  and  easily  handled  form.  To  a  certain  extent 
theories  are  given.  While  these  have  their  important  use, 
they  must  not  be  relied  upon  too  strongly.  "  Theories," 
says  Dumas,  "  are  like  crutches.  To  find  out  their  value 
we  must  try  to  walk  with  them."  On  the  other  hand, 
where  theories  have  been  presented  in  this  work,  effort  has 
been  made  to  show  distinctly  the  basis  upon  which  they 
rest.  Particularly  in  the  chapters  relating  to  atomic  weight 
the  attempt  has  been  made  to  lead  the  pupil  to  formally 
distinguish  between  facts  and  inferences. 

The  efforts  of  investigators  in  chemistry  as  in  other 
natural  sciences  are,  as  Berthelot  remarks,  to  transform  a 
mere  descriptive  science  into  a  truly  physical  and  mechan- 
ical, i.e.  a  mathematical,  one.  This  thought  has  not  been 
forgotten  in  arranging  the  work  presented  herewith.  At  the 
same  time  the  attempt  has  been  made  to  avoid  making  the 
book  mathematical.  The  mathematical  treatment,  valuable 

iii 

237484 


IV  PREFACE. 

as  it  is  for  highly  advanced  students,  is  apt   to  be  repellent 
to  beginners. 

The  author  takes  this  opportunity  to  thank  the  teachers 
of  the  United  States  for  the  kind  reception  they  have  given 
to  his  earlier  text-books  in  chemistry.  He  cherishes  the 
hope  that  they  may  find  this  one  of  service  to  them  in 
their  studies  and  their  teaching. 

BROWN  UNIVERSITY,  PROVIDENCE,  R.I., 
September,  1890. 


CONTENTS. 


PAGE 

CHAPTER  I.  —  The  branches  of  natural  science.  The  place  of  chem- 
istry   i 

CHAPTER  II.  —  The  constitution  of  matter.  The  atom.  The  molecule. 
(Elements  and  compounds.)  The  mass.  The  modern  atomic 
theory 3 

CHAPTER  III. —  Is  matter  indeed  molecular  and  atomic?  General 
discussion  upon  atoms  and  molecules.  Ordinary  observation. 
More  searching  examination.  Evidences  of  heterogeneity  found 
in  mechanical  and  physical  relations  of  substances;  in  their  rela- 
tions to  heat,  to  light,  and  to  the  electric  current;  and  in  their 
chemical  properties.  Compound  radicles.  General  conclusions. 
The  atoms  of  the  chemist  viewed  as  composite.  The  genesis  of 
atoms.  Shapes  of  atoms.  Movements  of  atoms.  Positions  of 
atoms  in  space 15 

CHAPTER  IV.  —  The  three  states  of  matter.  Solids,  liquids,  and  gases. 

Importance  of  the  study  of  gases.  Radiant  matter  .  .35 

CHAPTER  V.  —  Change  from  one  state  of  matter  to  another.  Influence 

of  addition  and  withdrawal  of  heat 42 

CHAPTER  VI.  —  Changes  incident  to  addition  of  heat.  Addition  of 
heat  to  a  solid.  Rise  of  temperature  according  to  specific  heat. 
Melting.  Latent  heat  of  liquefaction.  Special  forms  of  liquefac- 
tion. Dissociation.  Addition  of  heat  to  a  liquid.  Vaporization. 
Ice  machines.  Changes  incident  to  withdrawal  of  heat.  With- 
drawal of  heat  from  a  gas.  Withdrawal  of  heat  from  a  liquid. 
Solidification  of  homogeneous  and  of  mixed  liquids  .  .  -45 

CHAPTER  VII.  —  Certain  general  laws  of  matter.  Boyle's  or  Mariotte's 
law  of  the  pressure  of  gases.  Charles's  law  of  the  expansion  of 
gases  by  heat.  Graham's  two  laws  of  gaseous  diffusion.  The  law 
of  Henry  and  of  Dalton  of  the  relation  of  pressure  to  the  solu- 
bility of  a  gas  in  water.  The  law  of  definite  proportions.  The 

V 


VI  CONTENTS. 

PAGE 

two  laws  of  multiple  proportions.  Gay-Lussac's  three  laws  of 
combination  of  gases.  Avogadro's  and  Ampere's  hypothesis  of 
the  size  of  gaseous  molecules  .......  63 

CHAPTER  VIII.  —  Certain  forms  of  energy  closely  connected  with 
chemical  change.  Heat;  temperature;  expansion;  change  of 
state;  chemical  combination  and  decomposition;  light  (spec- 
trum analysis) ;  work.  Electricity :  its  sources  and  effects  .  .82 

CHAPTER  IX. — The  attractions  of  masses.     Gravitation      .         .         .99 

CHAPTER  X. — The  attractions  of  molecules.  I.  Cohesion.  In  solids; 
in  gases;  in  liquids.  Polarity;  crystallization;  cleavage.  Crystal- 
line systems.  The  process  of  crystallization  .  .  .  .  101 

CHAPTER  XI. — The  attractions  of  molecules  (continued}.  II.  Adhe- 
sion. (A}  Adhesion  between  solids  and  solids.  (B}  Adhesion 
between  solids  and  liquids;  moistening;  capillary  attraction; 
spheroidal  state;  solution  (deliquescence,  freezing  mixtures)  .  114 

CHAPTER  XII. — The  attractions  of  molecules  (continued}.  II.  Ad- 
hesion. The  separation  of  a  solid  from  a  liquid.  (C)  Adhesion 
between  solids  and  gases.  (D}  Adhesion  between  liquids  and 
liquids.  (E}  Adhesion  between  liquids  and  gases.  (F}  Adhe- 
sion between  gases  and  gases  (the  terrestrial  atmosphere)  .  .124 

CHAPTER  XIII. — The  attraction  of  atoms.  Chemical  affinity.  Con- 
ditions favoring  chemical  change:  the  liquid  condition;  heat 
(thermolysis  and  dissociation)  ;  light;  electricity;  vital  processes 
of  higher  and  lower  living  beings  (organic  and  inorganic  com- 
pounds) .  .  .  .  .  .  .  .  .  .  .141 

CHAPTER  XIV.  —  The  attraction  of  atoms  (continued}.  The  chem- 
ical work  of  micro-organisms.  Microbes :  their  conditions  of 
growth;  results  of  their  action;  their  usefulness  .  .  .  155 

CHAPTER  XV. — The  attraction  of  atoms  (continued}.  Modes  of 
chemical  action.  Sphere  of  chemical  action.  Criteria  of  chemical 
action.  Results  of  chemical  action.  General  laws  of  chemical  action,  168 

CHAPTER  XVI. — The  attraction  of  atoms  (continued}.  Thermo- 
chemistry :  its  laws  and  units.  Calorimeters  and  the  difficulties 
they  have  to  meet.  Range  of  thermo-chemistry.  Results  .  .  175 

CHAPTER  XVII.  —  The  attraction  of  atoms  (continued}.  Theories 

of  the  nature  of  chemical  attraction 187 

CHAPTER  XVIII. — Atomic  weight:  method  of  work  and  method  of 

description  .  .  .  .  .  .  .  .  .  .  .  193 


CONTENTS.  Vll 

PAGE 

CHAPTER  XIX.  —  Atomic  weight  (continued}.  First  step:  A  unit 
adopted.  Second  step :  Selection  of  the  compounds  and  the 
processes  to  be  employed 199 

CHAPTER  XX.  —  Atomic  weight  {continued}.  Third  step:  Experi- 
mental work  for  securing  a  few  atomic  weights.  Study  of  chlo- 
rine, bromine,  and  iodine;  sodium,  potassium,  and  silver  .  .  204 

CHAPTER  XXI.  —  Atomic  weight  (continued}.  Fourth  step:  The 
choice  of  a  particular  atomic  weight  from  several  combining  num- 
bers. Density  of  elementary  gases.  Volume  composition  of  com- 
pound gases.  Vapor  density  of  compound  substances.  Specific 
heats  of  elements  and  of  compounds.  Atomic  heats.  Specific 
heats  of  chlorine,  bromine,  iodine,  potassium,  sodium,  silver.  A 
study  of  oxygen  and  of  sulphur 209 

CHAPTER  XXII. — Atomic  weight  (continued}.  Fifth  step:  Con- 
firmation of  the  atomic  weights  chosen.  Molecular  formula  sup- 
ported by  volume  composition,  by  chemical  substitution,  by  melt- 
ing-points and  boiling-points,  by  crystalline  form,  by  molecular 
stability,  by  relationship,  by  results  of  decomposition,  by  excep- 
tional compounds,  by  special  properties  of  substances  .  .  .  225 

CHAPTER  XXIII.  —  Atomic  weight  {contimted}.  Sixth  step:  Bring 
all  the  atomic  weights  into  one  table.  (The  periodic  law.)  The 
work  of  Newlands,  Mendeleeff,  and  Carnelley.  Prout's  hypothesis,  243 

CHAPTER  XXIV.  —  Atomic  weight  (continued}.  Elementary  sub- 
stances as  molecular 249 


CHAPTER    I. 

THE   BRANCHES  OF  NATURAL  SCIENCE. 

THE   PLACE   OF   CHEMISTRY. 

THERE  may  be  as  many  sciences  as  there  are  kinds  of 
subject-matter  for  scientific  treatment. 

The  scientific  treatment  of  a  subject  demands  exact 
observation,  precise  description  with  fixed  nomenclature, 
classified  arrangement,  rational  explanation. 

The  term  natural  science  is  usually  applied  to  the 
classified  knowledge  of  external  material  nature  and 
certain  of  its  forces. 

In  the  ordinary  every-day  use  of  language,  then,  the 
general  term  science  is  often  applied  to  what  is  here  in- 
cluded in  the  term  natural  science.  But  it  must  not  be  for- 
gotten that  there  may  be  a  science  of  the  human  mind,  for 
example,  as  well  as  sciences  of  external  forms  of  matter. 

Divisions  of  Natural  Science.  —  One  grand  division  of 
natural  science  is  that  called  Natural  History.  In  this 
are  included  — 

Geology,  a  history  of  the  inanimate  matter  of  the  earth, 
and  embracing  physical  geography  and  meteorology  ; 

Zoology,  a  history  of  animals  ; 

Botany,  a  history  of  vegetable  beings. 

Natural  history  is  mainly  descriptive. 

Another  grand  division  of  natural  science  is  that  called 


;  BRANCHES  ,  QF    NATURAL    SCIENCE, 
v 

Natural  Philosophy,  or  Physical  Science.  In  this  are 
included  — 

Mechanics,  which  treats  of  masses  of  matter ; 

Physics  proper,  which  treats  of  the  motions  of  mole- 
cules, and  the  molecular  forces  such  as  light,  heat,  and 
electricity  ; 

Chemistry,  which  treats  of  atoms,  the  constitution  and 
properties  of  molecules,  and  the  laws  of  chemical  change. 

Physical  science  is  mainly  explanatory. 

Defects  of  the  Foregoing  Classification.  —  Even  a  very 
hasty  consideration  of  this  brief  classification  shows  that, 
especially  as  respects  exact  lines  of  demarcation,  it  is 
inadequate.  Thus  the  individuals  discussed  under  each 
department  of  natural  history  involve  in  their  histories 
the  processes  of  natural  philosophy;  for  the  animal, 
the  plant,  and  the  rock  are  formed,  or  live,  or  grow,  or 
merely  exist  in  one  place,  as  the  case  may  be,  under  condi- 
tions involving  mechanical,  physical,  and  chemical  forces. 

Again,  it  will  be  noted  that  certain  branches  of  study 
evidently  belonging  to  natural  science  —  astronomy,  for 
example — are  not  specifically  mentioned. 

But  the  defects  of  classifications  of  this  sort  are  refer- 
able to  difficulties  that  nature  itself  places  in  our  way, 
for  the  multitude  of  natural  phenomena  are  not  in  them- 
selves characterized  by  strongly  marked  division  lines, 
but  are  mostly  intimately  interwoven. 

Finally,  it  is  not  intended,  in  this  chapter,  to  offer  a 
perfect  classification  of  the  subjects  of  study  afforded 
by  natural  objects  and  forces  ;  it  is  merely  proposed  to 
place  before  the  reader  the  general  relations  of  chemistry 
to  other  natural  sciences. 


CHAPTER    II. 
THE   CONSTITUTION  OF  MATTER. 

THE   ATOM  ;     THE   MOLECULE  ;     THE    MASS. 

MATTER  is  believed  to  be  capable  of  existing  in  por- 
tions of  three  different  grades  of  magnitude.  These  are 
called  respectively  the  atom,  the  molecule,  the  mass. 

The  Atom.  —  An  atom  is  the  smallest  unit  of  matter 
now  recognized  as  existing.  About  seventy  different 
kinds  of  atoms  are  now  known. 

Each  atom  of  matter  is  viewed  as  possessing  the  fol- 
lowing characteristics,  in  addition  to  many  others  :  — 

It  is  extremely  small  (but  not  infinitely  small). 

It  is  indivisible,  and,  indeed,  in  itself  unchangeable. 

It  possesses  a  definite  weight,  which  may  be  determined  relatively  and 
absolutely. 

(The  atomic  weight  is  different  for  different  kinds  of  atoms,  but  practi- 
cally the  same  for  atoms  of  the  same  kind.  Thus  each  atom  of  hydrogen 
weighs  I  microcrith ;  each  atom  of  oxygen  weighs  about  16  microcriths. 
See  p.  199.) 

It  is  capable  of  manifesting  an  attractive  force,  called  chemical  affinity. 

It  almost  invariably  exists  in  a  group,  of  which  the  component  atoms 
may  be  alike  or  may  be  unlike.  In  a  very  few  cases  an  atom  may  exist 
singly. 

Examples  of  single  atoms  are  :  — 

C  Barium  (Ba), 
)    Cadmium  (Cd), 
i     Mercury  (Hg)  {Hydrargyrum), 
Zinc  (Zn). 


4  THE    CONSTITUTION    OF    MATTER. 

The  Molecule. — A  molecule  is  the  smallest  particle 
of  any  substance  that  manifests  the  chemical  properties 
of  that  particular  substance.  Thus  :  — 

H4C  represents  one  molecule  of  marsh  gas. 

H3N  "  "  "          "  ammonia  gas. 

H2O          "  "  "          "  water. 

H2  "  "  "          "  hydrogen. 

O3  "  "  "          "  ozone. 

O2  "  "  "          "  ordinary  oxygen. 

A  molecule  is  believed  to  be  capable  of  possessing 
most  of  the  following  characteristics,  in  addition  to 
many  others :  — 

(#)  It  is  extremely  small.  From  recent  physical  investigations,  "  it  may 
be  concluded  with  a  high  degree  of  probability  that  in  ordinary  liquids  or 
solids  the  diameter  of  the  molecule,"  that  is,  the  distance  between  the 
centres  of  contiguous  molecules,  is  between  the  one  two-hundred-and-fifty- 

millionth  („. inL^u.)  and  the  one  five-thousand-millionth  (,,^^1™)  of  an 

\  &>,.), 000,000  /  \o,uuu,ooo,uuo/ 

inch. 

(^)  It  is  not  completely  nor  absolutely  in  contact  with  its  neighboring 
molecules,  but  is  separated  from  them  by  relatively  large  spaces. 

(V)  When  in  the  state  of  gas  a  molecule  demands  the  same  amount  of 
space  as  every  other  molecule  in  the  gaseous  state  (under  the  same  condi- 
tions of  temperature  and  pressure). 

(</)  A  molecule  usually  consists  of  a  group  of  atoms.  These  atoms  are 
bound  together  by  chemical  affinity  and  into  a  union  of  exceedingly  inti- 
mate relationship.  In  fact,  in  most  cases  the  molecule  cannot  be  divided 
without  absolutely  changing  the  identity  of  the  substance.  This  result  might 
be  expected  in  the  case  of  molecules  composed  of  different  kinds  of  atoms. 

Thus  water  has  molecules,  each  expressible  by  the  formula  H2O.  When 
water  is  decomposed,  the  change  takes  place  as  follows :  — 

2  HO.  when  decomposed,  yield          2  H2  +  O2 

Two  molecules  of  Two  molecules  of  One  molecule  of 

Water,  Hydrogen  gas,  Oxygen  gas, 

36  4  32 

parts  by  weight.  parts  by  weight.  parts  by  weight. 


THE    CONSTITUTION    OF    MATTER.  5 

But  though  at  first  unexpected,  it  is  easily  seen  that  a  change  of  iden- 
tity may  result  from  the  decomposition  or  rearrangement  even  of  molecules 
whose  individual  atoms  are  of  precisely  the  same  kind.  Thus  ozone  has 
molecules,  each  expressible  by  the  formula  O.j.  When  ozone  is  decomposed, 
the  change  takes  place  as  follows :  — 

2  O3  yield  by  decomposition  3  O2 

Two  molecules  of  Three  molecules  of 

Ozone,  Ordinary  Oxygen, 

96  96 

parts  by  weight.  parts  by  weight. 

Now  it  is  an  observed  fact  that  the  properties  and  powers  of  ordinary 
oxygen  are  very  different  from  those  of  ozone. 

(^)  The  term  molecule,  usually  reserved  for  groups  of  atoms,  is  ex- 
tended to  single  atoms  in  the  four  exceptional  cases,  already  cited,  in  which 
the  single  atom  is  capable  of  existing  apart.  Thus  a  molecule  of  mercury 
means  also  a  single  atom  of  mercury. 

Elements  and  Compounds.  —  Matter  is  called  simple,  or 
elementary,  when  its  molecules  are  composed  of  atoms 
of  the  same  kind. 

Most  of  the  elementary  gases  are  believed  to  have  two 
atoms  in  the  molecule. 

Thus,  the  hydrogen  molecule  is  expressed 

H  -  H   or  H2, 
and  the  oxygen  molecule, 

O  =  O  or  O2. 

Matter  is  called  compound  when  its  molecules  are 
made  up  of  different  kinds  of  atoms.  The  number  of 
atoms  in  many  compound  molecules  is  but  two  ;  in 
others  it  is  greater,  sometimes  reaching  to  several 
hundreds. 

Examples :     Starch,         Cr,H10O5, 

21  atoms. 

Protagon,     C^H^PO^, 

434  atoms. 


6  THE    CONSTITUTION    OF    MATTER. 

The  Mass.  —  A  mass  of  matter  is  a  collection  of  mole- 
cules. Portions  of  matter  appreciable  by  the  senses 
are,  in  most  cases,  masses. 


FIG.  i.  —  Antoine  Laurent  Lavoisier.     Born  in  Paris,  August  26,  1743;  died  on  the 
scaffold  in  Paris,  May  8,  1794. 


The  Modern  Atomic  Theory. — The  views  of  the  con- 
stitution of  matter  set  forth  in  the  foregoing  paragraphs 
may  be  said  to  have  their  origin  in  the  atomic  theory  of 
Dr.  John  Dalton,  an  English  mathematician,  who  began 
to  develop  his  theory  in  the  year  1803. 

In    some    ancient    metaphysical    speculations    matter 


THE    CONSTITUTION    OF    MATTER.  7 

was  held  to  be  infinitely  divisible,  while  in  others  the 
contrary  view  was  maintained. 

The  ancient  philosophers  who  constructed  atomic 
theories  felt  sure,  upon  general  grounds,  that  matter  is 
susceptible  of  division  to  a  degree  far  beyond  that  which 


FIG.  2. —  Chemical  balance.     (A  portion  is  shown  cut  away  so  as  to 
display  the  tripod.) 

their  appliances  effected.  But  when  the  question  arose, 
"  Is  not  matter  then  infinitely  divisible  ? "  no  valid  answer 
could  be  given.  An  unconquerable  difference  of  opinion 
existed. 

The  ancient  views  were  defective  because  they  were 
too  largely  speculative  and  were  not  based  on  a  suffi- 
ciently large  number  of  facts  —  especially  such  facts  as 
can  be  learned  only  by  carefully  devised  and  conducted 
experiments. 


8 


THE    CONSTITUTION    OF    MATTER. 


The  modern  chemist  declares  that  in  fact  there  exist 
at  present  limits  to  the  divisibility  of  matter.  More- 
over, Dalton's  theory,  as  well  as  other  modern  views  of 
the  constitution  of  matter,  are  based  not  upon  specula- 


FIG.  3. —  Simple  form  of  barometer,  called  Torricelli's.  The  tube  A  is  closed  at  the 
top.  The  mercury  does  not  fall  because  of  the  atmospheric  pressure  exerted  upon  the 
surface  of  mercury  at  MN. 

tion,  but  upon  discoveries  reached  by  accurate  experi- 
ments with  carefully  devised  appliances. 

The  most  important  and  suggestive  facts  were  col- 
lected by  the  chemists  of  the  last  part  of  the  eighteenth 
century,  and  especially  Lavoisier.  He  led  the  way  by 


FIG.  4.  —  Special  barometer  used  for  measuring  the  heights  of  mountains.  The  tripod 
is  constructed  so  that  it  may  be  closed,  and  thus  protect  the  more  delicate  parts  of  the 
barometer.  Upon  ascending  a  mountain  the  diminished  atmospheric  pressure  is  mani- 
fested by  the  fall  of  mercury  in  the  long  tube  of  the  barometer. 


FIG.  5.  — Water  barometer  in  the  Tour  Saint- Jacques,  Paris.  Of  course  the  tube  is 
much  longer  than  a  mercurial  barometer.  On  the  other  hand,  the  variations  in  the  height 
of  the  barometer  are  much  more  marked.  A  variation  of  atmospheric  pressure  represent- 
ing an  inch  on  the  mercurial  barometer  produces  a  variation  of  about  13.6  inches  in  the 
water  barometer. 


Fig.  6.  — Larger  view  of  the  base  of  the  water  barometer  shown  in  section  in  Figure  5. 
The  self-registering  apparatus  containing  clock-work  and  paper  cylinder  is  shown  at  the 
right. 


12  THE    CONSTITUTION    OF    MATTER. 

his  rigid  use  of  the  balance  in  chemical  investigation. 
He  maintained  —  what  was  not  perceived  before  —  that 
weight  is  an  important  function  of  matter,  and  a  safe 
guide  in  chemical  reasoning.  His  teaching  showed  the 
value  of  those  quantitative  methods  which  have  not  only 
afforded  a  sure  basis  for  modern  theories  of  matter,  but 
have  been  a  most  important  aid  in  the  general  modern 
progress  of  chemistry. 

The  first  notions  of  the  modern  atomic  theory  appear 
to  have  been  suggested  to  Dalton  by  following  La- 
voisier's methods ;  i.e.  by  the  use  of  the  quantitative 
processes  of  the  time.  Dalton's  work  led  also  to  the 
enunciation  of  the  important  laws  of  definite  proportions 
and  of  multiple  proportions,  —  laws  which  are  still  the 
chief  support  of  that  theory. 

Modern  instruments  of  precision,  notably  the  balance, 
the  barometer,  the  graduated  eudiometer,1  and  the  ther- 
mometer have  afforded  large  contributions  toward  new 
and  exact  knowledge  of  matter  and  its  forces ;  they  have 
also  aided  to  dispel  many  old  and  false  impressions. 

"  The  vicissitudes  in  the  fortunes  of  a  truly  scientific  idea  are  aptly  illus- 
trated by  the  history  of  the  atomic  theory.  After  a  period  of  dormancy  of 
more  than  2000  years  the  atomic  theory  was  revived  and  rendered  definite 
by  Dalton,  was  firmly  established  on  an  experimental  basis  by  Berzelius, 
was  almost  abandoned  by  the  school  founded  by  the  same  chemist,  was 
rehabilitated  and  again  nearly  despaired  of  by  Dumas,  was  largely  advanced 
by  Avogadro,  was  subdivided  and  its  parts  clearly  distinguished  by  Gerhardt 
and  Laurent,  and  is  now  the  foundation-stone  of  a  great  and  ever-increasing 
edifice."2 

Atoms  and  molecules  are  now  considered  to  be  real  existences  as  truly 
as  are  planets  and  fixed  stars;  and  they  are  as  truly  susceptible  of  measure- 

1  Eudiometer:  an  instrument  or  vessel  for  exact  measurement  of  the  volume 
of  a  gas. 

2  Muir,  Principles  of  Chemistry,  p.  24. 


1 709 


FIG.  7.  —  Portion  of  a  barometer,  show- 
ing al  the  top  a  part  of  the  long  glass  tube 
containing  mercury.  The  reservoir  of  mer- 
cury in  the  tube  may  be  protected  from 
undue  agitation  by  advancing  the  screw  Q 
so  that  its  plug  may  close  the  lower  aper- 
ture of  the  long  glass  tube. 


FIG.  8.  —  Top  of  mercury  column  of  a 
barometer.  The  rack  and  pinion  move- 
ment is  for  the  purpose  of  moving  the 
vernier  in  order  to  afford  more  accurate 
reading. 


14  THE    CONSTITUTION    OF    MATTER. 

ment.     Their  motions,  though  of  a  different  kind,  are  as  much  matters  of 
fact. 

It  must  be  admitted  that  even  to-day  some  persons  question  the  exist- 
ence of  the  chemical  atoms.  The  difficulty  is  probably  the  outcome  of 
ambiguity  in  the  word  atom.  The  atom  of  the  Greek  atomists  may  or  may 
not  be  the  atom  of  the  modern  chemist.  The  Greek  atom  is  something 
that  by  its  very  nature  cannot  ever  be  divided.  The  atom  described  by 
modern  scientists  is  merely  a  very  minute  portion  of  matter  that  has  not 
yet  been  divided.  The  Greek  atom  is  a  metaphysical  creation;  the  atom 
of  modern  science  is  a  real  thing. 


CHAPTER    III. 
IS  MATTER  INDEED   MOLECULAR  AND  ATOMIC? 

EVIDENCE   POINTING  TO   CERTAIN   DEGREES  OF   HETERO- 
GENEITY. 

General  Discussion  upon  Atoms  and  Molecules.  —  Proba- 
bly many  persons  —  and  especially  those  whose  reason- 
ing powers  are  but  imperfectly  developed  —  would  have 
more  confidence  in  the  existence  of  atoms  and  molecules 
if  they  were  visible. 

But  after  all,  the  senses  are  by  no  means  infallible 
guides.  The  sense  of  sight  often  deceives  ;  even  in  the 
commonest  affairs  of  life  reliance  is  often  placed  upon 
the  reasoning  powers  in  opposition  to  the  direct  evi- 
dence of  the  eye.  Especially  when  the  eye  is  diseased, 
it  gives  false  ideas  of  color,  form,  and  the  like.  Even 
excellent  eyes  may  not  be  able  to  see  the  vibration  of  a 
certain  piano-string  while  it  is  singing ;  but  by  applica- 
tion of  processes  of  reasoning  to  experimental  results 
proof  can  be  obtained,  to  the  satisfaction  of  every  one,  of 
the  existence  of  such  vibration.  It  is  only  by  processes 
of  reasoning  that  the  distances  of  the  heavenly  bodies 
are  measured,  but  no  one  doubts  the  data  that  astrono- 
mers furnish. 

However,  single  atoms  and  single  molecules  are  not 
visible,  and,  moreover,  it  does  not  seem  likely  that  opti- 
cal appliances  will  ever  extend  the  range  of  man's  visual 

'5 


16       IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ? 

powers  to  such  minute  objects.  But  neither  these  diffi- 
culties nor  any  others  need  deter  the  'investigator  from 
efforts  to  solve  the  problem  of  the  constitution  of  matter. 
He  may  with  propriety  continue  to  ask  such  general 
questions  as  the  following  :  Is  matter  constructed  on 
the  type  of  an  imperforate  mass,  or  is  it  more  truly  a 
network  of  some  sort  ?  Does  one  minute  portion  of  a 
given  kind  of  substance  come  into  absolute  contact  with 
its  neighboring  portions,  so  that  no  pointed  implement, 
however  fine  and  sharp,  can  discover  places  of  more  and 
of  less  resistance,  or  are  there  minute  avenues  of  some 
sort  affording  more  easy  passage  in  some  places  than  in 
others  ?  Is  matter  indeed  atomic  and  molecular,  as  has 
been  declared  ? 

The  questions  come  to  this  :  Is  matter  heterogeneous  ? 
If  the  answer  is  yes,  the  next  questions  are  as  to  the 
character  and  extent  of  this  heterogeneity. 

These  questions  do  indeed  receive  decisive  answers  from  physical  and 
chemical  facts.  By  proper  reasoning  from  natural  phenomena  there  have 
been  secured,  at  first  approximate,  and  later  highly  exact,  answers.  A 
blind  man  may  discover  the  construction  of  a  gate  that  bars  his  path.  By 
applying  to  such  an  obstacle  instruments  of  various  shapes  and  sizes  he 
may  be  able  to  state  whether  it  has  the  apparent  continuity  of  sheet-iron, 
or  has  horizontal  or  vertical  bars,  or  is  of  netted  gauze,  or  even  has  perfo- 
rations like  fine  card-board.  In  like  manner  scientific  observations  may  be 
made  as  to  the  action  of  matter  in  response  to  certain  general  experiments. 
Then  proceeding  on  and  on,  there  may  be  secured  a  tolerably  certain 
knowledge  of  its  constitution. 

Ordinary  Observation  upon  this  Subject.  —  Some  forms 
of  matter  show  plainly  to  the  eye  that  they  are  made  up 
of  more  than  one  kind  of  substance  —  that  they  are 
heterogeneous.  Other  kinds  may  appear  to  the  eye  to  be 
homogeneous,  while  a  fuller  knowledge  of  them  makes  it 


IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC?        \"J 

absolutely  certain  that  they  are  not  so.  As  a  very  sim- 
ple example  consider  sugar.  It  appears  at  first  consid- 
eration to  be  alike  all  through ;  it  is  very  easy  to  prove, 
however,  that  it  contains  at  least  carbon  —  something 
very  different  from  sugar. 

More  Searching  Examination.  —  There  is  presented 
below  a  very  brief  resume  of  certain  observed  properties 
or  actions  of  matter  that  point  first  to  general  hetero- 
geneity or  grained  structure  of  some  sort,  and  next 
to  distinct  molecular,  and  finally  to  distinct  atomic 
composition. 

The  facts  stated  here  are  by  no  means  all  that  apply. 
The  whole  body  of  physical  and  chemical  knowledge 
points  in  this  direction. 

FIRST.  Evidences  of  heterogeneity  found  in  certain 
mechanical  properties  of  substances. 

(a)  The   diminution  in   volume  of   solids,   of  liquids, 
and  —  in    a   yet    more    marked    degree  —  of   gases,    by 
mechanical  pressure,  leads  to  the  suggestion  that  there 
are    interior    spaces    which    are    diminished    by    that 
action. 

(b)  In  similar  manner  peculiarities  of  internal  struc- 
ture are  revealed  by  certain  changes  of  the  outline  of 
bodies.     The  various  degrees  of  -elasticity,  hardness,  mal- 
leability, and  ductility  displayed  by  various  solids  show 
various  kinds  of  heterogeneity. 

The  changes  of  thickness  of  soap-bubble  films,  before  they  break,  have 
been  carefully  studied.  The  relations  of  these  films  to  light  show  their 
thickness  at  different  times.  The  thinnest  films  appear  to  be  commen- 
surable with  the  diameters  of  a  molecule,  as  learned  by  other  methods. 


1 8   IS  MATTER  INDEED  MOLECULAR  AND  ATOMIC? 

(c)  Crystals  possess  cleavage  and  certain  other  prop- 
erties which  offer  most  marked  suggestions  of  internal 
peculiarities  of  structure. 

The  foregoing  classes  of  facts  point,  therefore,  to 
general  heterogeneity  of  substances  in  their  internal 
make-up. 

SECOND.  Evidences  of  heterogeneity  found  in  certain 
physical  properties  of  substances. 

(a)  Gases  diffuse   readily  into  other  gases.      Liquids 
readily    absorb    gases.       Certain    solids    swallow    large 
quantities  of   gases  without  corresponding   increase  of 
bulk.      (Palladium  and  platinum  occlude  hydrogen  in  an 
especially  noteworthy  degree.) 

(b)  Liquids  diffuse  themselves  into  other  liquids  with 
great   rapidity,   sometimes   (as   in   case   of   alcohol   and 
water)  with  diminution  of  volume. 

(c)  Solids  dissolve  quickly  in  liquids  without  unusual 
external  influence,  diffusing  themselves  throughout  the 
solvents.     (Dialysis  is  a  specialized  form  of  the  motion 
of  solids  into  liquids.) 

Some  solids  seem  to  dissolve  in  gases,  rising  up  into 
those  gases  under  certain  conditions,  at  temperatures  far 
below  those  at  which  these  solids  ordinarily  volatilize.1 

These  classes  of  facts  point  to  internal  places  of  feeble 
resistance  in  substances.  Thus  they  point  to  molecular 
grouping  of  the  firmer  portions. 

THIRD.  Evidences  of  heterogeneity  found  in  the  rela- 
tions of  certain  substances  to  heat. 

1  Hannay  and  Hogarth, 


IS  MATTER  INDEED  MOLECULAR  AND  ATOMIC?   1 9 

(a)  Solids,  liquids,  and  gases  expand  upon  addition  of 
heat  and  contract  upon  its  withdrawal.  The  details  of 
these  operations  need  not  be  presented  here.  But  the 
fact  that  different  gases  expand  equally  with  heat  is  a 


FIG.  9.  —  The  flask  contains  a  dark-colored  liquid,  which  does  not  allow  the  light  to 
pass.  It  is  diathermanous,  however,  for  heat  rays  go  through  it  easily,  as  may  be  demon- 
strated by  concentrating  them  upon  a  piece  of  phosphorus  or  other  combustible  material. 

special  case  under  this  head,  and  it  points  to  similarity 
of  arrangement  of  particles  in  gases. 

The  fact  that  some  crystals  expand  more  in  certain  directions  than  in 
others  adds  force  to  the  argument;  so  do  the  facts  connected  with  changes 
of  substances  in  what  may  be  called  an  upward  direction  from  solids  to 


2O       IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ? 

liquids  and  gases,  and  downward  from  gases  to  liquids  and  solids,  respec- 
tively by  addition  or  withdrawal  of  heat. 

The  continuity  of  the  three  states  of  matter  as  shown  by  Thomas 
Andrews  furnishes  additional  general  evidence  of  molecular  structure  in 
matter.  (See  p.  35.) 

(b]  When  certain  substances  are  highly  heated,  they 
acquire  anomalous  vapor  densities.     These  point  to  dis- 


FIG.  10.  —  Arrangement  for  showing  the  refraction  of  light  by  water.  A  beam  of 
light,  coming  through  the  aperture,  penetrates  the  water  in  the  jar,  but  is  bent  out  of  its 
original  course. 

sociation  of  molecules  (and  thence  to  the  existence  of 
molecules). 

(c)  All  substances  when  highly  heated  give  out  light. 
In  most  cases  a  given  metal,  for  example,  gives  out  light 
made  up  of  waves  having  many  different  rates  of  vibra- 
tion. This  fact  is  proved  by  the  spectroscope. 


IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC?       21 

Moreover,  the  same  substance  may  give  different 
colors  (and  so  different  spectra)  at  different  high  tem- 
peratures. 

These  facts  seem  to  prove  that  an  apparently  homo- 
geneous substance  may  possess  different  internal  vibrat- 
ing parts.  (See  p.  86.) 

(d}  Certain  bodies  manifest  in  a  remarkable  degree 
the  phenomena  associated  with  specific  heat  and  latent 
heat.  (See  pp.  45  and  47.)  Again,  as  respects  radiant 


FIG.  ii.  —  Crystal  of  Iceland  spar,  showing  double  refraction;   the  single  line  is  made  to 
appear  double. 

dark  heat,  some  substances  are  distinctly  diatherma- 
nous,  while  others  are  athermanous.  These  peculiari- 
ties cannot  be  discussed  at  length  here.  But  they  are 
explicable  only  upon  the  theory  that  the  substances  dis- 
playing them  are  molecular.  Thus  they  support  the 
proposition  under  review. 

FOURTH.      Evidences   of  heterogeneity  found  in    the 
relation  of  certain  substances  to  light. 


22      IS    MATTER   INDEED    MOLECULAR   AND    ATOMIC  ? 

The  optical  properties  of  substances  afford  some  of 
the  most  clear  and  decisive  evidences  of  grainedness  of 
structure  in  bodies.  The  rays  of  light  appear  to  be 
capable  of  use  as  a  penetrating  agency  of  a  most  dis- 
criminating character. 

It  is  not  practicable  here,  however,  to  do  more  than  allude  to  the  classes 
of  optical  facts  referred  to. 


FIG.  12. —  Method  of  displaying  phenomena  of  double  refraction.  A  single  beam  of 
light  passed  through  a  double-refracting  crystal  appears  on  the  screen  as  two  beams 
of  light. 


The  opacity,  the  translucency,  or  the  transparency  of  a  given  body;  the 
phenomena  of  ordinary  and  of  double  refraction ;  the  polarization  of  light, 
and  the  passage  of  the  polarized  ray  through  crystals  in  certain  positions 
and  not  in  others;  the  peculiar  passage  of  the  polarized  ray  also  through 
solids  under  pressure  as  compared  with  the  same  at  normal  conditions; 
the  rotation  of  the  polarized  ray  by  sugar  solutions  and  the  like,  —  all  con- 
tribute greatly  to  substantiate  the  molecular  theory.  (In  case  of  certain 
substances,  as  cane  sugar,  the  greater  the  quantity  of  the  substance,  the 
greater  the  rotation.  This  suggests  some  greater  interference  of  a  greater 
number  of  molecular  particles.) 


IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ?       23 

FIFTH.     Evidences  of  heterogeneity  found  in  the  rela- 
tions of  certain  substances  to  the  electric  ctirrent. 

(a)  In  some  cases  elongation  takes  place  when  sub- 
stances are  rendered  magnetic  by  an  electric  current. 
Thus  iron  bars  and  steel  bars  are  so  elongated. 

(b)  A  certain  substance  (boro-silicate  of  lead)  trans- 
mits polarized  light  when  electrified,  otherwise  not. 


FIG.  13.  —  Polarizing  saccharimeter.  A  ray  of  polarized  light  passing  through  a  solu- 
tion of  cane  sugar  in  water  is  rotated  in  such  a  way  as  to  enable  the  observer  to  determine 
the  percentage  of  sugar  in  the  sample. 

(c)  Certain  rarified  gases  and  vapors   show  peculiar 
stratification  when  made  luminous  by  the  electric  dis- 
charge of  the  Ruhmkorff  coil.     The  stratification  is  dif- 
ferent for  different  gases.     This  indicates  the  existence 
of  internal  portions  in  these  substances. 

(d)  A  given   portion   of  oxygen  gas  is  changed  by 
the  silent  electric  discharge  into  ozone  without  loss  of 
weight,  but  with  diminution  of  volume. 


24   IS  MATTER  INDEED  MOLECULAR  AND  ATOMIC 


(e)  The  phenomena  of  electrolysis  are  very  striking. 
A  given  battery  current  passing  simultaneously  through 
several  compounds  in  different  vessels  may  decompose 
them  all  at  once.  The  weights  of  certain  elements  thus 
separated  are  very  different.  The  following  examples 
may  be  given  :  — 


FIG.  14.  —  A  portion  of  gas  appears  at  first  homogeneous,  but  is  shown  in  disconnected 
strata  when  under  the  influence  of  an  electric  spark  furnished  by  a  Ruhmkorff  coil.  The 
coil  is  connected  with  a  single  cell  of  a  bichromate  battery.  The  glass  tube  containing 
the  gas  is  called  a  Geissler  tube. 


Hydrogen 
Copper  . 
Zinc 


2.0  parts  by  weight. 
63.2      "      " 
64.9      "      " 


The  amounts  are  closely  proportioned  to  the  amounts 
separated  by  similar  chemical  operations.  These  and 
other  similar  numbers  represent  a  tendency  to  subdi- 
vision by  different  but  definite  quantities.  Thus  they 


IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC?       25 

show  definite  internal  structure  in  masses  of  the  sub- 
stances mentioned. 

General  Comment  on  the  Foregoing  Evidences.  —  Heat,  and 
especially  light  and  electricity,  are  agencies  possessing  an  extraordinary 
power  of  penetrating  bodies.  So  far  as  they  penetrate  them  with  greater 
or  less  ease  in  certain  directions  or  under  special  conditions,  they  cannot 
fail  to  raise  the  suggestion  that  the  bodies  in  question  are  not  perfectly 
homogeneous,  but  possess  spaces  of  greater  or  less  resistance.  But  this  is 
what  was  to  be  demonstrated  by  the  foregoing  paragraphs. 


FIG.  15.  — Geissler  tube,  showing  that  a  gas  appearing  at  first  to  be  homogeneous 
manifests  stratification  when  illuminated  by  an  electric  current  from  a  Ruhmkorff  coil. 

SIXTH.  Evidences  of  heterogeneity  found  in  the  chem- 
ical relations  of  substances. 

The  chemical  suggestions  of  molecular  structure  are 
far  more  distinct  and  precise  than  the  mechanical  and 
physical.  These  may  now  be  discussed.  Of  course 
only  a  few  will  be  presented.  If  completeness  were 
sought,  the  entire  body  of  chemical  knowledge  would 
have  to  be  adduced.  The  whole  of  modern  chemistry 
must  be  referred  to  as  the  argument  in  this  part  of 
the  case. 


26      IS    MATTER    INDEED    MOLECULAR   AND    ATOMIC  ? 

Illustration.  —  Chemical  analysis  shows  that  most  sub- 
stances are  made  up  of  more  than  one  kind  of  matter. 
With  the  exception  of  about  seventy  substances  called 
elements,  everything  known  has  internal  complexity 
instead  of  internal  uniformity  throughout. 

Thus  the  purest  cooking  salt,  apparently  homoge- 
neous, is  really  made  up  of  two  very  different  sub- 
stances or  kinds  of  matter,  —  chlorine  and  sodium. 

The  chlorine  has  certain  properties  —  distinctly  its 
own  —  of  which  may  be  mentioned  its  gaseous  form, 
its  green  color,  and  its  bleaching  power.  And  simi- 
larly, the  sodium  has  certain  properties  —  distinctly  its 
own  —  of  which  may  be  mentioned  its  metallic  lustre,  its 
affinity  for  oxygen,  and  its  power  to  decompose  water 
in  a  certain  way. 

By  analysis  pure  salt  may  be  subdivided  into  portions 
of  these  two  very  different  kinds  of  matter. 

And  by  synthesis  chlorine  and  sodium  may  be  made 
to  combine,  and  when  they  do  so  combine,  they  form  a 
new  product,  which  is  precisely  common  salt  and  noth- 
ing else. 

But  the  new  product  is  not  a  mere  mixture  of  its  constituents. 

(a)  The  constituents  are  more  thoroughly  intermingled  or  interdiffused 
than  is  possible  in  mere  mixtures.  For  the  minutest  portions  that  can  be 
recognized  as  containing  true  salt  contain  both  of  the  factors,  and  not  a 
single  one. 

(£)  The  constituents  are  associated  in  fixed  and  definite  proportions  by 
weight  (always  23  parts  of  sodium  to  35.4  parts  of  chlorine),  —  facts  of 
which  mere  mixtures  are  independent. 

(c)  The  constituents  cannot  be  separated  by  any  mechanical  processes 
nor  by  any  processes  other  than  chemical  or  physico-chemical. 

(</)  The  new  product,  the  common  salt,  has  no  one  of  the  six  proper- 
ties heretofore  enumerated  as  possessed  by  the  constituents  (nor  yet  has  it 
the  average  of  these  properties)  :  instead,  it  has  a  new  set  of  properties 
entirely  its  own. 


IS  MATTER  INDEED  MOLECULAR  AND  ATOMIC  ?   2/ 

It  is  plain  that  a  mass  of  common  salt  is  made  up  of 
small  portions  of  salt,  each  small  portion  containing  a 
definite  amount  of  chlorine  and  of  sodium.  These  small 
portions  are  the  units  already  called  molecules.  Each 
of  these  molecules  is  made  up  of  heterogeneous  parts. 
But  this  is  the  point  that  was  to  be  demonstrated. 

Practically  the  same  method  of  reasoning  may  be 
applied  to  every  other  substance  known,  except  the 
seventy  elementary  ones.  But  further,  a  careful  study 
of  the  elementary  substances  has  shown  that  these  are 
composed  of  the  little  groups  called  molecules,  only  in 
their  cases  the  heterogeneity  of  parts  is  far  less  marked 
in  quantity  and  quality,  though  apparently  equally 
marked  in  a  reality  of  distinct  portions. 

Compound  Radicles.  —  Sometimes  a  grotfp  of  atoms  is 
recognized  as  capable  of  transfer  as  a  whole  from  one 
complex  molecule  to  another,  and  yet  not  capable  of 
existing  alone.  To  such  a  group  the  name  compound 
radicle  is  applied.  A  simple  example  is  found  in  the 
hypothetical  metal  ammonium  (NH4).  Thus  the  com- 
pound called  sal-ammoniac  (ammonic  chloride,  NH4C1), 
contains  the  group  NH4,  which  is  capable  of  continued 
transfer,  as  a  whole,  from  one  molecule  to  another ;  it 
acts  very  much  like  an  element — only,  if  liberated,  it 
at  once  decomposes.  Chemists  know  a  large  number 
of  other  compound  radicles  of  similar  type  ;  indeed,  a 
whole  class  of  bodies  called  compound  ammonias  has 
been  studied  very  thoroughly  by  Hofmann  and  others. 

Conclusions  Reached.  —  There  can  be  no  reasonable 
doubt  then  that  there  exist  molecules  —  in  the  general 


28       IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ? 

sensn  of  small  portions  of  matter  that  preserve  an  integ- 
rity tJirough  a  series  of  operations.  They  are  in  fact 
units  of  a  certain  order. 

Further,  there  can  be  no  doubt  that  the  molecular 


FIG.  16.  —  August  Wilhelm  Hofmann,  Professor  of  Chemistry  in  the  University 
of  Berlin. 

units  are  made  up  of  units  of  another  order,  —  the 
so-called  elementary  atoms.  These  units  are  also  inte- 
gral parts,  and  they  preserve  their  unbroken  composition 
—  whatever  it  may  be  —  in  spite  of  any  processes  as  yet 
applied  to  them.  Whether  or  not  they  are  real  atoms, 


IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ?        2Q 

in  the  sense  of  absolutely  indivisible  things,  is  not  posi- 
tively known.  There  are  certain  grounds  for  believing 
tJiat  they  are  not.  That  they  are  units  of  a  most  impor- 
tant character  must  be  true.  Their  recognition  is  the 
work  of  modern  investigators.  Their  discovery  is  a  most 
valuable  and  fruitful  one,  whatever  theory  of  their  ulti- 
mate composition  any  one  may  hold,  or  whatever  new 
facts  relating  to  them  may  be  discovered  hereafter. 

Are  the  Atoms  of  the  Chemist  Composite  ?  —  The  no- 
tion has  often  been  presented  that  the  seventy  substan- 
ces called  elementary  are  in  fact  very  stable  compounds 
not  yet  decomposed.  Chemists,  physicists,  and  philoso- 
phers have  explicitly  declared  their  belief  in  this  view. 
Dalton,  Faraday,  Gladstone,  Brodie,  Graham,  Mills, 
Stokes,  Lockyer,  and  other  experimenters  have  dis- 
tinctly held  it.  From  another  side,  thinkers  like  Her- 
bert Spencer  have  recognized  the  necessity  of  it. 

Four  general  grounds  for  believing  the  elements  to  be  composite  may 
be  stated  here  :  — 

FIRST.  Lockyer  considers  that  the  fact  that  a  given  element  affords  a 
multitude  of  spectrum  lines  at  once  suggests  that  these  lines  are  somehow 
connected  with  different  vibratory  powers  of  different  portions  of  the  atom. 
In  other  words,  the  atom  is  composite. 

SECOND.  Dr.  Carnelley  offers  a  strong  argument  from  the  analogies  of 
certain  acknowledged  compounds.  It  is  this:  There  are  recognized,  in 
the  group  of  hydrocarbons,  a  continuous  series  of  compounds  advancing  in 
molecular  weights  by  regular  numerical  steps,  and  having,  with  these,  a 
regularly  advancing  sequence  of  physical  and  chemical  properties.  So  the 
elements,  as  arranged  by  Newlands,  Mendeleeff,  Lothar  Meyer,  and  others, 
constitute  a  series  of  substances  whose  members  show  a  regular  advance  in 
atomic  weights,  and  with  it  a  regular  progress  of  physical  and  chemical 
properties.  There  is  no  doubt  that  the  hydrocarbons  are  compounds; 
there  is  then  a  high  probability  that  the  elements  are  so  also.  Carnelley 
goes  so  far  as  to  offer  the  theory  that  the  seventy  elements  are  composed 
by  union,  in  different  ways,  of  three  kinds  of  ultimate*. 


3O       IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ? 

THIRD.  Professor  Crookes  considers  that  he  has  in  certain  cases 
actually  decomposed  certain  elements.  He  says  that  the  substances  he 
has  obtained  from  so-called  "old  yttria"  "are  not  impurities  in  yttrium. 
They  constitute  a  veritable  splitting  up  of  the  yttrium  molecule  into  its 
constituents." 

FOURTH.  Certain  so-called  elements,  as  tellurium  and  didymium,  have 
been  considered  compounds  on  grounds  based  on  their  general  chemical 
properties.1 

The  Genesis  of  Atoms.  —  Professor  Crookes  proposes 
the  theory  that  matter,  as  we  now  know  it,  has  been  pro- 
duced by  a  kind  of  evolution  from  an  antecedent  sub- 
stance or  fire-mist  which  he  calls  protyle.  He  considers 
that  by  a  series  of  large  falls  of  temperature  alternating 
with  periods  of  approximately  stationary  temperature, 
the  protyle  has  become  condensed  or  combined  into  a 
series  of  substances  —  our  elements. 

"  The  first-born  element  would,  in  its  simplicity,  be  most  nearly  allied  to 
protyle.  This  is  hydrogen,  of  all  known  bodies  the  simplest  in  structure 
and  of  the  lowest  atomic  weight. 

"  Any  well-defined  element  may  be  likened  to  a  platform  of  stability, 
connected  by  ladders  of  unstable  bodies.  In  the  first  coalescence  of  the 
primitive  stuff  there  would  be  formed  the  smallest  atoms;  these  would  then 
unite,  forming  larger  groups;  the  gaps  between  the  several  stages  would 
gradually  be  bridged  over,  and  the  stable  element  appropriate  to  that  stage 
would  absorb,  so  to  speak,  the  unstable  rungs  of  the  ladder  which  led  up 
to  it. 

"  In  this  genesis  of  the  elements  the  longer  the  time  taken  up  in  the 
cooling- down  process,  during  which  the  hardening  of  protyle  into  atoms 
takes  place,  the  more  sharply  defined  would  be  the  resulting  elements; 
whilst  the  more  rapid  and  the  more  irregular  the  cooling,  the  more  closely 
the  resulting  bodies  would  fade  into  each  other  by  almost  imperceptible 
degrees.  Thus  we  may  conceive  that  the  succession  of  events  which  gave 
rise  to  such  groups  as  platinum,  osmium,  and  iridium  —  palladium,  ruthe- 
nium, and  rhodium  —  iron,  nickel,  and  cobalt  —  might  have  produced  only 
one  element  in  each  of  these  three  groups  if  the  process  had  been  greatly 

1  Brauner,  J.  Ch.  Soc.,  1889,  p.  382. 


IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC?       3! 

prolonged.  And  conversely,  had  the  rate  of  cooling  been  much  more  rapid, 
elements  might  have  originated  still  more  nearly  identical  than  are  nickel 
and  cobalt.  Thus  may  have  arisen  the  closely  allied  elements  of  the  cerium, 
yttrium,  and  similar  groups.  In  fact,  we  may  regard  the  collocation  of  the 


FIG    17.  —  Sir  William  Thomson,  distinguished  English  electrician  and  physicist. 


minerals  of  the  class  of  samarskite  and  gadolinite  as  a  kind  of  cosmical 
lumber-room,  where  elements  in  a  state  of  arrested  development  —  the  un- 
connected missing  links  of  inorganic  Darwinism  —  are  gathered  together." 

Shapes   of  Atoms.  -  -  Sir  William  Thomson  has  pro- 
pounded the  theory  that  atoms  possess  the  shape  of 


32       IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ? 

rings  of  various  kinds.  In  this  view  each  ring  pos- 
sesses a  vortex  motion,  something  like  that  produced 
by  the  spontaneous  ignition  of  bubbles  of  phosphuretted 
hydrogen. 

Movements  of  Atoms.  —  It  is  at  present  unknown 
whether  there  is  any  constant  motion  of  atoms  within 
the  molecule  unattended  by  decomposition  of  it.  Gen- 
eral analogy  and  certain  facts  strongly  favor  the  affirma- 
tive view.  There  is  plenty  of  room  in  gaseous  molecules 
at  least. 

Of  course  when  a  molecule  is  decomposed  there  is 
generally  necessity  of  movement  of  its  atoms ;  but  this 
kind  of  motion  may  be  only  temporary,  dependent  upon 
the  duration  of  the  particular  exciting  cause. 

Relative  Position  of  Atoms  in  Space.  —  The  brilliant 
studies  of  Kekule  upon  organic  compounds  have  given 
rise  to  pretty  general  adoption  of  a  certain  expression 
for  the  substance  benzol  (C6H6).  This  takes  the  form 
of  a  hexagon  called  the  benzol  hexagon  or  the  benzol 
ring,  as  represented  by  the  diagram  below  :  — 

I 

C 

//    \ 
-C         C- 

I       II 

-C         C- 

^  / 

C 


In  this  expression  the  six  atoms  of  carbon  are  viewed 
as  forming  a  closed  chain,  of  which  each  atom  is  attached 


34       IS    MATTER    INDEED    MOLECULAR    AND    ATOMIC  ? 

to  its  neighboring  atoms,  on  the  one  hand  by  two  points 
and  on  the  other  hand  by  one  point  of  attraction. 

Recent  studies  of  van  't  Hoff,  Pasteur,  Wislicenus,  and  others  have  car- 
ried the  subject  still  further.  Attempting  to  explain  certain  remarkable 
peculiarities  of  carbon  compounds,  the  theory  has  now  been  presented  that 
the  four  points  of  attraction  of  a  single  atom  of  carbon  are  ordinarily  dis- 
tributed about  its  central  point  somewhat  as  are  the  four  apexes  of  the 
regular  tetrahedron.  Again,  when  the  series  of  carbon  atoms  are  attached 
in  circuit,  it  is  supposed  that  in  some  cases  one  carbon  atom  is  related  to 
its  neighbor  as  if  the  apex  of  a  tetrahedron  were  presented  to  the  apex  of 
another  tetrahedron.  Again,  in  another  case,  a  carbon  atom  may  be 
attached  to  its  neighbor  as  if  an  edge  of  a  tetrahedron  were  presented  to 
the  edge  of  another  tetrahedron.  In  the  former  case  it  is  supposed  that 
the  one  carbon  atom  is  capable  of  a  certain  amount  of  rotation  about  the 
other  carbon  atom.  In  the  latter  case  such  rotation  would  naturally  be 
impracticable.  It  is  supposed  that  this  theory  contributes  something 
toward  explanation  of  asymetric  carbon  compounds;  such,  for  instance, 
as  the  different  varieties  of  tartaric  acid. 


CHAPTER    IV. 
THE   THREE   STATES  OF  MATTER. 

SOLIDS,   LIQUIDS,   AND   GASES. 

THE  various  kinds  of  matter  known  are  capable  of 
arrangement  in  a  series  representing  many  stages  of 
tenuity.  Between  the  most  rigid  solids  on  the  one 
hand,  and  the  most  rarified  gases  on  the  other,  there 
exist  substances  which  possess  many  degrees  of  den- 
sity, the  progress  from  one  extreme  to  the  other  being 
exceedingly  gradual. 

There  are  even  no  well-defined  lines  of  demarcation 
between  a  solid  and  its  own  liquid,  and  between  a  li- 
quid and  its  own  vapor.  Indeed,  Thomas  Andrews  has 
shown  that  in  case  of  the  same  substance  there  may  be 
even  a  continuity  of  its  liquid  and  gaseous  form.  Thus 
water  may  exist  in  a  closed  vessel  under  such  condi- 
tions, at  once  of  high  temperature  and  pressure,  that 
the  upper  portions  shall  be  gaseous  and  the  lower  por- 
tions liquid,  and  yet  the  latter  may  show  no  top  surface 
or  other  evidence  of  a  distinct  place  where  liquid  ends 
and  vapor  begins. 

For  purposes  of  convenience,  however,  chemists  admit 
the  existence  of  three,  and  perhaps  four,  states  of  mat- 
ter. They  are  called  respectively  the  solid,  the  liquid, 
the  gaseous,  and  the  radiant  ftfrms. 

From  what  has  just  been  said,  however,  it  is  evident 
that  it  is  not  easy  to  define  the  four  terms  used. 

35 


36  THE    THREE    STATES    OF    MATTER. 

A  Solid  Defined.  —  In  general,  a  solid  is  a  form  of 
matter  possessing  such  rigidity  that  a  given  portion  of 
it  resists  with  considerable  vigor  any  forces  that  tend 
to  change  its  shape.  Of  course,  then,  a  given  solid  does 
not  —  unless  under  very  great  pressure  —  assume  the 
shape  of  a  vessel  in  which  it  is  placed,  and  except,  of 
course,  the  one  was  contrived  to  fit  the  other,  or  the 
solid  is  in  the  form  of  powder.  (Possibly  the  latter 
qualification  is  unnecessary.) 

A  Liquid  Defined. —  In  general,  a  liquid  is  a  fluid  form 
of  matter  having  such  internal  mobility  as  enables  it  to 
assume  the  form  of  at  least  the  lower  portions  of  its 
containing  vessel,  whatever  the  shape,  and  provided 
only  that  the  liquid  shall  be  in  sufficient  quantity. 
Further,  an  untrammelled  liquid  has  usually  a  hori- 
zontal top  surface. 

A  Gas  Defined.  —  In  general,  a  gas  is  a  fluid  form  of 
matter  such  as  readily  (under  ordinarily  prevailing  con- 
ditions) assumes  the  shape  of  the  containing  walls  of 
any  vessel  whatsoever.  Far  from  having  any  distinct 
top  surface  of  its  own,  a  gas  has  sufficient  expansive 
force  to  lead  it  to  fill  entirely  any  containing  vessel 
whatever  its  size. 

One  of  the  most  characteristic  features  of  bodies  when  in  the  gaseous 
state  is  the  great  change  in  volume  which  they  may  be  easily  made  to 
undergo.  Their  volumes  are  particularly  sensitive  to  change  of  pressure 
and  of  temperature.  When  the  gas  is  heated,  great  expansion  takes  place 
if  the  walls  of  the  vessel  are  such  as  permit  it.  (If  expansion  is  fully  re- 
sisted, however,  there  is  still  an  increase  of  tension  or  expansive  force  im- 
parted to  the  gas.  Hence  addition  of  heat  may  produce  increase  cf  volume 
of  a  gas  when  pressure  is  maintained  constant,  or  it  may  produce  increase 
of  tension  when  volume  is  maintained  constant.) 


THE    THREE    STATES    OF    MATTER.  37 

Andrews  recommends  that  the  term  gas  be  reserved  for  a  substance 
when  it  is  at  a  temperature  higher  than  its  critical  point.  He  recommends 
that  the  term  vapor\)Z  applied  to  a  substance,  —  in  the  gaseous  condition,  — 
but  existing  at  a  temperature  below  its  critical  point. 

The  term  vapor  is  applied,  in  a  general  way,  to  those  gases  which  are 
easily  condensible  to  the  liquid  form. 

The  modern  scientist  accepts  the  hypothesis  that  even 
a  given  small  portion  of  gaseous  matter  is  made  up  of 
millions  of  millions  of  molecules,  and  that  these  are  in 
rapid  motion  in  all  directions.  In  all  ordinary  gases  the 
length  of  the  mean  free  path  of  a  molecule  is  exceed- 
ingly small  as  compared  with  the  dimensions  of  the 
confining  vessel ;  in  fact,  in  every  second  of  time  each 
molecule  has  millions  of  collisions  with  other  molecules. 
Indeed,  many  of  the  well-recognized  properties  of  gas- 
eous matter  are  referable  to  these  constant  encounters. 

Importance  of  the  Study  of  Gases.  —  Several  of  the  gen- 
eral laws  of  matter  relate  to  it  while  in  its  gaseous  condi- 
tion. They  show  that  bodies  of  the  most  diverse  chemical 
constitution  and  properties,  possess  many  close  physical 
correspondences  when  in  the  gaseous  form,  while  the 
solid  and  liquid  forms  possess  no  equivalent  resemblances. 

These  facts  create  the  belief  that  there  is  some  funda- 
mental similarity  and  simplicity  of  molecular  condition 
characterizing  the  gaseous  state.  Hence  this  condition 
has  been  very  carefully  studied  as  that  affording  to 
molecular  science  the  best  common  ground  for  the  com- 
parison of  bodies.  The  hypothesis  of  Avogadro  and 
Ampere  is  a  result  of  this  view.  (See  p.  78.) 

It  is  probable,  however,  that  a  given  substance  may  possess  in  the  solid 
condition  and  in  the  liquid  condition  molecules  containing  a  greater  num- 
ber of  atoms  than  it  has  while  in  the  gaseous  condition. 


38  THE    THREE    STATES    OF    MATTER. 

Thus  it  has  been  shown  by  Raoult  and  by  Thomsen,  from  very  dis- 
similar points  of  view,  that  the  molecular  formula  of  liquid  -water  is  proba- 
bly as  great  as  H4O2.  (This  does  not  invalidate  the  statement  of  the  formula 
of  water  vapor  as  H2O.) 

It  is  probable  that  the  systems  of  molecules  in  liquids  and  solids  are  not 
only  very  different  from  those  of  gases,  but  that  they  vary  greatly  among 
themselves.  These  molecular  aggregations,  too,  are  probably  not  perma- 
nent, but  are  continually  breaking  up,  their  constituents  changing  partners. 


FIG.  19.  —  Crookes's  radiant  matter  bulb.  At  b  is  a  screen.  When  the  electric  cur- 
rent traverses  the  radiant  matter  in  the  bulb,  the  molecules  fall  against  the  screen  b\ 
thereupon  they  are  intercepted,  and  a  dark  shadow  is  thrown  at  d. 


Radiant  Matter.  — The  radiant  form  of  matter  is  much 
more  tenuous  than  even  the  gaseous,  which  it  most  resem- 
bles. In  the  radiant  form  the  molecules  are  very  far  apart 
compared  with  their  distances  in  the  gaseous  form.  It 
is  this  circumstance  which  leads  them  to  display  physical 
properties  different  from  those  of  the  same  bodies  in  the 
gaseous  condition.  As  a  result,  also,  the  mean  free 
path  is  rendered  so  long  that  the  molecular  hits  in  a 
given  time  may  be  disregarded  in  comparison  with  the 
misses ;  thus  each  molecule  is  allowed  more  fully  to 
obey  its  own  laws  without  interference.  When  the  free 


THE    THREE    STATES    OF    MATTER. 


39 


path  becomes  so  long  as  to  be  comparable  with  the 
dimensions  of  the  vessel,  the  properties  which  charac- 
terize the  gaseous  condition  are  reduced  to  a  minimum, 


FIG.  20.  —  Appearance  of  the  illuminated  screen  and  the  dark  image  formed  in 
Crookes's  bulb,  shown  in  Figure  13. 

and   the   matter  becomes   exalted  to   the    ultra-gaseous 
state  called  the  radiant  condition. 

The  study  of  radiant  matter  has  been  conducted  of 
late  years  with  great  earnestness  and  success  by  Pro- 


FIG.  21. —  Crookes's  bulb.  The  bulb  contains  a  small  amount  of  gas  in  the  radiant 
form.  When  the  electric  current  is  passed  through  the  tube,  the  molecules  of  gas  strike 
against  the  paddles,  setting  the  wheel  in  motion  and  making  it  traverse  the  track  from 
one  end  of  the  bulb  to  the  other. 

fessor  William  Crookes  of  London.  His  experiments 
have  been  conducted  in  glass  tubes  or  bulbs  from  whose 
interiors  the  major  portion  of  gas  originally  within  them 


4O  THE    THREE    STATES    OF    MATTER. 

has  been  removed.  The  minute  portions  remaining  are 
then  in  the  radiant  form.  Professor  Crookes  has  investi- 
gated the  properties  of  radiant  matter  by  the  use  of 
these  tubes  and  certain  ingenious  mechanical  appliances 
constructed  within  them.  He  seems  to  have  demon- 


FIG.  22.  —  Crookes's  radiometer.  The  bulb  contains  a  very  small  amount  of  gas  in 
the  radiant  form.  Under  the  influence  of  a  very  minute  amount  of  heat,  the  radiant 
matter  causes  the  vanes  of  the  mill  to  rotate  rapidly. 

strated  a  number  of   propositions  like  the  following  : 

(1)  The  length  of  the  mean  free  path  of  molecules  of 
matter    in    the    radiant    condition    can    be    measured ; 

(2)  radiant  matter  exerts  powerful  phosphorogenic  ac- 
tion where  it  strikes ;    (3)  radiant  matter  proceeds  in 
straight  lines  —  it  will  not  turn  a  corner ;   (4)  radiant 


THE    THREE    STATES    OF    MATTER.  4! 

matter  when  intercepted  by  solid  matter  casts  a  shadow  ; 
(5)  radiant  matter  exerts  strong  mechanical  action  where 
it  strikes  —  this  is  proved  by  various  forms  of  the  well- 


FIG.  23.  —  Crookes's  bulb.  The  bulb  contains  a  small  amount  of  gas  in  the  radiant 
form.  Under  ordinary  conditions,  even  when  the  electric  current  is  flowing  through  the 
bulb,  the  gas  is  intercepted  by  the  screen  cd.  When,  however,  the  magnet^-  is  placed  as 
indicated,  the  molecules  are  attracted  so  that  they  flow  over  the  screen.  They  strike  the 
paddles  of  the  mill-wheel,  setting  it  in  motion. 

known  apparatus  called  Crookes's  radiometer  ;  (6)  radi- 
ant matter  is  deflected  by  a  magnet ;  (7)  radiant  matter 
produces  heat  when  its  motion  is  arrested. 


CHAPTER    V. 
CHANGE   FROM  ONE   STATE   TO  ANOTHER. 

INFLUENCE  OF  ADDITION  AND  OF  WITHDRAWAL  OF  HEAT. 

Introduction. —  Of  the  seventy  chemical  elements,  five 
are  gaseous  at  ordinary  temperatures,  —  hydrogen,  nitro- 
gen, oxygen,  fluorine,  and  chlorine.     Two  are  liquids,— 
bromine  and  mercury.     The  others  are  solids. 

The  terrestrial  globe  (with  its  oceans  and  atmos- 
phere) contains  a  very  large  number  of  compound  sub- 
stances. Under  the  conditions  of  temperature  pre- 
vailing, the  majority  of  them  are  solid ;  a  considerable 
number  exist  in  the  atmosphere  in  the  gaseous  form  ; 
there  is  one  widespread  liquid,  — water.  (The  water  of 
the  ocean,  it  is  true,  holds  in  solution  a  good  many  sub- 
stances which  may  be  considered  to  exist  —  for  the  time 
being  —  in  the  liquid  form.) 

Chemists  have  recognized  an  enormous  number  of 
compound  substances  other  than  those  existing  in  na- 
ture. Of  these,  some  are  solid  at  ordinary  temperatures, 
some  are  liquid  at  ordinary  temperatures,  some  are  gas- 
eous at  ordinary  temperatures. 

Influence  of  Heat.  —  In  general,  the  condition  of  a 
substance,  whether  solid,  liquid,  or  gaseous,  is  mainly 
dependent  upon  the  amount  of  heat  it  contains.  Of 

course   the   foregoing   discussion    has   dealt   with    sub- 
42 


CHANGE    FROM    ONE    STATE    TO    ANOTHER. 


43 


stances  under  ordinary  conditions  of  temperature  and 
under  the  possession  by  each  individual  substance  of 
such  an  amount  of  heat  as  they  may  take  up  from  the 
terrestrial  system  of  earth,  water,  and  air. 

But  chemists  are  able  to  add  to  or  subtract  from  substances  a  very  large 
amount  of  heat  as  compared  with  what  the  substances  absorb  from  their 
natural  surrounding  medium.  Incidentally  the  temperature  of  the  sub- 


FIG.  24.  —  Disposition  of  apparatus  for  determining  the  melting-point  of  a  solid.  At 
the  moment  the  solid  liquefies  electric  communication  is  established  through  the  wires 
and  a  bell  is  rung.  At  the  moment  of  ringing  the  temperature  is  read  off  from  the 
thermometer. 


stance  is  raised  or  lowered  by  addition  or  withdrawal  of  heat  respectively, 
although  not  in  exact  proportion  to  the  amount  added  or  withdrawn.  The 
actual  change  of  temperature  is  modified  somewhat  by  certain  natural 
properties  of  the  substance,  which  give  rise  to  the  phenomena  of  specific 
heat.  (See  p.  45.) 

The  addition  of  heat  to  a  body  tends  also  to  advance  it  from  the  solid  to 
the  liquid,  and  even  to  the  gaseous  state.  And  likewise  the  withdrawal  of 
heat  from  a  body  tends  to  reduce  it  from  the  gaseous  to  the  liquid  and 
even  to  the  solid  state.  The  word  tends  is  used  here  with  the  admission 
that,  while  in  some  cases  the  tendency  is  realized  and  the  result  sought  is 


/j/|      CHANGE  FROM  ONE  STATE  TO  ANOTHER. 

attained,  in  others  it  is  not.  In  a  few  of  the  latter  cases  the  requisite 
amounts  of  heating  or  cooling  influences  are  not  obtained  by  means  at 
present  at  the  command  of  the  chemist.  In  many  cases  dissociation  inter- 
feres. (See  p.  49.) 

A  Double  Change  of  State.  —  Certain  well-known  substances  may 
be  made  at  will,  by  addition  or  subtraction  of  moderate  quantities  of  heat, 
to  occupy  either  the  solid,  the  liquid,  or  a  gaseous  state.  Sulphur  and 
phosphorus  are  examples  of  such  elementary  substances,  and  water  is  an 
example  of  such  a  compound  substance. 

It  is  not  easy,  however,  to  give  many  familiar  examples  of  this  sort. 

A  Single  Change  of  State.  —  There  are,  however,  many  well-known 
cases  of  substances  ordinarily  solid  that  may  be  changed  to  the  liquid  state. 

Thus,  probably  every  one  of  the  chemical  elements,  solid  at  ordinary 
temperatures,  may  be  liquefied  (carbon,  perhaps,  excepted).  Again,  the 
two  liquid  elements,  bromine  and  mercury,  may  be  vaporized.  Further, 
the  gaseous  elements  may  all  be  liquefied  and  probably  even  solidified. 

Most  compound  substances,  whether  natural  or  artificial,  which  are  solid 
at  ordinary  temperatures  of  the  earth,  may  be  liquefied;  and  in  most  cases 
these  liquids  —  and  also  compound  substances  liquid  at  ordinary  tempera- 
tures—  may  be  vaporized.  An  important  exception  must  of  course  be 
made  of  that  large  class  of  .organic  substances  (especially  the  organized 
ones),  which,  ordinarily  solid,  undergo  dissociation  —  and  that  without 
liquefying  —  by  addition  of  a  moderate  amount  of  heat.  The  same  state- 
ment applies  also  to  certain  substances  which  may  become  liquid,  but  dis- 
sociate upon  heating,  before  changing  to  the  state  of  vapor. 

In  general,  however,  the  chemist  assumes  that  there  are  conditions  — 
perhaps  not  always  easy  to  secure  —  under  which  every  substance,  with  the 
general  exceptions  noted,  may  be  made  to  assume  the  solid,  the  liquid,  or 
the  gaseous  state. 


CHAPTER   VI. 
CHANGES  INCIDENT  TO  ADDITION  OF  HEAT. 

ADDITION   OF   HEAT  TO   A   SOLID. 

WHEN  heat  is  added  to  a  solid,  a  series  of  effects  may 
be  produced  in  more  or  less  orderly  sequence,  somewhat 
as  follows  :  (i)  rise  of  temperature  according  to  the 
specific  heat  of  the  substance  ;  (2)  melting,  associated 
with  disappearance  of  heat  under  the  conditions  called 
latent  heat ;  (3)  a  series  of  effects  upon  the  liquid  as, 
for  example,  rise  of  temperature  according  to  the  speci- 
fic heat  of  the  liquid  ;  (4)  partial  vaporization  followed 
by  boiling  of  the  liquid;  (5)  complete  change  of  a  liquid 
to  the  form  of  vapor ;  (6)  incidental  to  many  of  these 
stages  is  expansion,  whether  of  the  solid,  the  liquid,  or 
the  gas ;  (7)  dissociation  may  take  place  at  different 
points  of  temperature  according  to  the  nature  of  the 
substance,  i.e.  in  some  cases  a  solid  may  dissociate,  in 
other  cases  a  liquid  may  dissociate,  and  in  yet  others 
dissociation  may  only  take  place  after  the  substance  has 
attained  the  gaseous  form  and  the  gas  has  been  sub- 
jected to  an  extremely  large  addition  of  heat ;  (8)  fin- 
ally evolution  of  light  may  occur. 

Rise  of  Temperature  according  to  Specific  Heat. — The 

specific  heat  of  a  substance  is   the  special   amount  of 
heat   involved   in   its   rise   or  fall   of   temperature.     To 

45 


46 


CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 


change  a  pound  of  water  one  degree  upward  in  temper- 
ature, a  large  amount  of  heat  must  be  added  to  it.  To 
change  a  pound  of  water  one  degree  downward  in  tem- 
perature, a  large  amount  of  heat  must  be  subtracted 
from  it.  Most  other  substances  have  lower  specific 
heats  than  water;  i.e.  a  smaller  amount  of  heat,  added  or 
subtracted,  changes  the  temperature  of  a  pound  of  any 
substance  other  than  water  by  one  degree  upward  or 
downward  respectively. 


TABLE   OF  A   FEW   SPECIFIC   HEATS. 


ATOMIC  WEIGHT. 
Hydrogen  =  i. 

SPECIFIC  HEAT. 
Water  =  i. 

Sodium  

23- 

•293 

Potassium    

39- 

.166 

Iron 

CCO 

112 

Copper 

6l2 

•  y  j 

Bromine  (solid)    

7Q.8 

.084. 

Silver      

IO7  7 

OC7 

Iodine     .                     .          . 

1266 

OC4. 

Platinum 

IQ4.  4 

O72 

Mercury  (liquid)       .... 

199.7 

•033 

Mercury  (solid)    

199.7 

.032 

Uranium      

238.5 

.028 

Gradual  Melting.  —  In  some  cases  of  addition  of  heat 
to  a  solid,  liquefaction  seems  to  be  a  gradual  process, 
and  the  temperature  of  the  solid  steadily  rises.  Sealing- 
wax  illustrates  this  case.  It  becomes  softer  and  softer 
until  distinctly  liquid.  This  kind  of  melting  is  most 
common  in  substances  of  decidedly  heterogeneous  com- 
position. 


CHANGES    INCIDENT    TO    ADDITION    OF    HEAT.  4/ 

Partial  Melting.  —  In  other  cases  addition  of  heat  fully 
melts  a  portion  of  the  mass  of  solid,  leaving  the  rest 
hard.  Little  by  little  the  whole  liquefies. 

This  kind  of  melting  is  most  common  in  bodies  of 
uniform  structure  and  marked  crystalline  tendencies. 

Disappearance  of  Heat  during  Liquefaction  (Latent  Heat  of 
Liquefaction) .  —  A  most  important  feature  in  these  latter  cases  is  the 
fact  that,  from  the  beginning  of  liquefaction  to  its  completion,  there  is  no 
rise  of  temperature.  Large  amounts  of  heat  may  thus  be  rendered  latent 
without  effect  upon  the  temperature  of  the  mass.  Ice  is  an  example  of 
this  process. 

When  ice  of  the  temperature  of  o°  C.  receives  such  an  addition  of  heat 
as  will  but  just  liquefy  it,  the  water  so  produced  does  not  rise  in  tempera- 
ture, but  still  remains  at  o°.  In  fact,  one  kilogramme  of  ice  absorbs,  while 
melting,  79  units  of  heat  without  any  rise  of  temperature  at  all.  The  heat 
so  absorbed  is  called  latent  heat.  That  it  has  merely  undergone  some 
temporary  modification  appears  from  the  fact  that  when  water  is  solidified 
into  ice  it  always  gives  out  79  units  of  heat;  in  other  words,  the  precise 
quantity  it  previously  disposed  of.  Moreover,  the  water  makes  this  latter 
change  without  any  fall  of  its  temperature. 

In  cases  like  the  foregoing,  the  heat  which  does  not  increase  the  tem- 
perature is  supposed  to  do  some  kind  of  internal  work  in  changing  the 
structure  or  positions  of  molecules. 

In  general,  a  definite  amount  of  heat  becomes  latent  when  any  solid  is 
changed  to  the  liquid  form,  and  it  reappears  when  the  liquid  changes  back 
again  to  the  solid  form. 

That  solids  in  changing  to  the  liquid  form  do  indeed  absorb  heat  is 
easily  proved  in  cases  like  that  of  snow  or  ice  melting  in  the  hand. 

Exceptional  Cases.  —  There  are  certain  exceptional 
cases  in  which  a  solid,  when  heated,  turns  so  quickly  to 
the  gaseous  form  that  it  seems  as  if  it  did  not  exist  in 
the  liquid  form  at  all.  Solid  iodine  is  an  illustration. 

Experiment.  —  Heat  about  one  gramme  of  iodine  crystals  in  a  large 
flask,  with  the  intention  of  liquefying  it  if  possible.  It  may  be  noted  that 
the  substance  turns  quickly  to  gas,  apparently  without  liquefying  at  all. 


48  CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 

Observe  also  that  the  iodine  vapor  condenses  in  the  upper  part  of  the  flask 
immediately  to  the  solid  form.  If  it  condensed  to  the  liquid  form,  it  might 
be  expected  to  form  in  drops  or  run  down  in  streams,  as  many  other  con- 
densing gases  and  vapors  do. 

NOTE.  Remove  the  iodine  from  the  flask  as  follows :  Introduce  a  few 
c.c.  of  water  and  about  one-half  gramme  of  solid  potassic  iodide.  Let 
the  solution  flow  to  different  parts  of  the  flask  so  as  to  dissolve  the 
iodine. 

Spontaneous  Melting  in  the  Air.  —  Certain  solids  melt 
upon  exposure  to  air  of  the  ordinary  temperature.  Ice  is 
an  excellent  example.  It  melts  by  the  addition  of  heat 
from  the  atmosphere  or  from  any  surrounding  solid 
or  liquid  bodies.  The  surrounding  substances  are  of 
course  cooled  by  the  process.  Many  cooling  operations 
in  the  arts  depend  upon  this  principle.  It  must  be 
observed,  however,  that  the  heat  lost  by  the  cooled 
objects  is  absorbed  by  the  melting  ice  as  latent  heat. 

Special  Forms  of  Liquefaction.  —  In  each  of  the  cases, 
previously  referred  to,  a  single  individual  substance  has 
been  assumed  to  liquefy  by  the  addition  of  heat.  There 
are  many  cases  in  which  two  substances  together  form 
a  single  liquid.  Thus  two  solids,  when  properly  inter- 
mingled, may  change  to  a  liquid.  For  example,  solid 
ice  and  solid  salt,  when  pulverized  together,  turn  into  a 
liquid.  Then  the  one  substance  dissolves  in  the  other  ; 
i.e.  the  solid  salt  dissolves  in  the  liquid  water. 

In  some  cases  a  solid  becomes  liquid  by  solution  in 
some  liquid.  Thus  liquid  water  and  solid  common  salt, 
when  intermingled,  may  form  a  single  liquid.  Of  course 
a  solid  salt  has  dissolved  in  the  liquid  water. 

Thus  ammonic  nitrate,  ammonic  acetate,  potassic 
iodide,  and  many  other  solids,  when  placed  in  a  small 


CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 


49 


amount   of   water,  dissolve  very  rapidly ;    at   the   same 
time  they  absorb  heat  to  a  most  marked  degree. 

In  all  such  cases  heat  is  absorbed.  It  may  be  called 
latent  heat  of  solution  as  distinguished  from  latent  heat 
of  liquefaction  already  referred  to. 

Other  Special  Cases.  —  Many 
special  cases  are  known  in  which  a  mix- 
ture of  two  or  more  solids  melts  at  a 
lower  temperature  than  that  at  which 
the  easiest-melting  one  melts  alone. 

Thus  certain  alloys  of  bismuth,  tin, 
lead,  and  cadmium,  called  fusible  al- 
loys, melt  at  the  temperature  of  about 
1 60°  Fahrenheit;  that  is,  fifty  degrees 
below  the  boiling-point  of  water.  But 
the  melting-point  of  tin  alone,  the 
easiest  melting,  is  451°  F.1 

So  also  certain  mixtures  of  chemi- 
cal salts,  called  salt  alloys,  illustrate 
the  same  principle  by  melting  at  a 
lower  point  than  either  salt  singly. 

These  and  other  somewhat  similar 
phenomena  are  classed  under  the  term 
eutexia.  An  eutectic  mixture  of  sub- 
stances is  that  mixture  which  pos- 
sesses a  lower  liquefying-point  than 
that  of  either  constituent  separately 
or  any  other  mixture  (not  chemical  compound)  of  them. 

These  phenomena  must  probably  be  referred  to  some  obscure  chemical 
combination  of  the  substances  involved.  (See  p.  172.) 

Dissociation.  —  In  case  of  certain  substances  dissocia- 
tion takes  place  at  a  temperature  far  below  that  at  which 

1  Melting-point  of  tin  alone 451°  F. 

"  bismuth  alone 515°  F. 

"  cadmium  alone 608°  F. 

"  lead  alone        619°  F. 


FIG.  25.  —  A  rod  of  fusible  alloy  is 
suspended  in  the  steam  rising  from  boil- 
ing water.  The  rod  melts  and  falls  in 
liquid  drops. 


5O  CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 

the  substances  become  luminous.  The  familiar  case  of 
mercuric  oxide  affords  an  example.  On  the  other  hand, 
some  substances  seem  never  to  be  decomposed  by  mere 
accession  of  any  such  quantity  of  heat  as  we  can  add  to 
them.  Silicic  oxide,  sand,  is  an  example. 

In  a  third  class  of  cases  dissociation  occurs,  but  it  is 
merely  temporary  ;  that  is,  if  immediately  after  dissocia- 
tion the  temperature  of  the  substances  falls  slightly,  the 
elements  recombine  so  that  the  original  compound  reap- 
pears as  if  it  had  never  been  decomposed  at  all.  For 
this  reason  many  of  the  facts  of  dissociation  long  escaped 
notice.  Of  late,  however,  new  methods  of  experimenting 
have  made  it  possible  to  prove  dissociation  in  cases  in 
which  it  was  at  first  unsuspected,  and  later  even  denied. 

When  dissociation  does  occur,  it  may  be  supposed  that  the  accession  of 
heat  has  become  so  great  as  to  produce  not  only  motion  of  the  molecules 
as  wholes,  but,  further,  to  produce  a  motion  of  the  atoms  within  the  mole- 
cules; in  its  earlier  stages  producing  some  of  the  phenomena  of  specific 
heat,  in  its  later  ones  the  heat  seems  to  increase  the  atomic  motion  until 
the  constituent  atoms  move  through  such  distances  as  throw  them  outside 
of  the  range  of  the  force  of  chemical  affinity.  Then  occurs  dissociation. 

It  may  be  added  parenthetically  that  in  the  view  of  certain  eminent 
investigators  this  line  of  research  suggests  the  probability  that  what  are 
commonly  regarded  as  ultimate  atoms  may  themselves  be  dissociated  by  a 
still  greater  heat.  In  this  view  the  number  of  true  elementary  substances 
is  much  smaller  than  is  now  admitted.  (See  p.  29.) 


ADDITION   OF   HEAT  TO  A   LIQUID. 

When  heat  is  added  to  a  liquid,  a  series  of  effects 
may  be  produced  in  more  or  less  orderly  sequence ;  they 
correspond  tolerably  well  to  those  associated  with  the 
addition  of  heat  to  a  solid.  They  are  somewhat  as  fol- 
lows :  (i)  rise  of  temperature  ;  (2)  expansion  ;  (3)  vapor- 


CHANGES    INCIDENT    TO    ADDITION    OF    HEAT.  5  I 

ization,  associated  with  disappearance  of  heat  as  latent 
heat  of  vaporization  ;  (4)  boiling ;  (5)  dissociation  may 
or  may  not  occur ;  (6)  if  the  addition  of  heat  is  contin- 
ued, evolution  of  light  may  be  produced. 


FIG.  26.  —  One  form  of  Carre's  ice  machine.  The  purpose  is  to  freeze  the  water  in  the 
carafe  D  '.  When  the  air-pump  is  set  in  motion,  the  air,  and  at  the  same  time  the  water 
vapor,  is  withdrawn  from  the  flask  D.  The  water  vapor  is  absorbed  by  concentrated 
sulphuric  acid  placed  in  the  reservoir  B.  This  absorption  is  rendered  the  more  prompt 
because  the  sulphuric  acid  is  agitated  by  a  plunger  in  C.  The  very  rapid  evaporation 
from  the  surface  of  the  water  in  D  absorbs  so  much  latent  heat  from  the  water  as  to 
freeze  it. 


Rise  of  Temperature.  —  When  heat  is  added  to  a  liq- 
uid, the  temperature  rises,  according  to  the  relations  of 
the  liquid  to  specific  heat,  until  the  boiling  temperature 


52  CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 

is  reached.  The  boiling-point  varies  not  only  for  differ- 
ent liquids,  but  varies  for  the  same  liquid  according  to 
the  pressure  at  the  time  prevailing. 

Vaporization  of  Liquids.  —  At  all  ordinary  tempera- 
tures a  liquid  gives  off  vapor  into  the  space  above  it. 
The  amount  of  this  vapor  depends  on  the  nature  of 


FIG.  27.  —  Small  form  of  Carre's  ice  machine.  Solution  of  ammonia  in  water  is  con- 
tained in  the  receiver  A.  Under  the  influence  of  heat,  ammonia  gas  is  driven  into  the 
jacket  in  B.  The  ammonia  gas  in  this  jacket  is  liquefied  by  reason  of  its.  own  pressure. 
In  the  second  stage  of  the  operation  the  heat  is  withdrawn  from  A,  whereupon  the  liquid 
ammonia  in  the  jacket  volatilizes  rapidly.  It  returns  to  the  water  in  A.  By  reason  of 
its  rapid  evaporation,  it  cools  that  portion  of  water  in  the  inner  cylinder  in  B.  The  water 
is  thereby  frozen. 

the  liquid  and  the  temperature  prevailing  at  the  time 
of  the  experiment.  The  vapor  exerts  a  pressure  called 
vapor  pressure.  At  first  vapor  comes  from  the  surface 
only.  As  the  temperature  rises,  more  and  more  vapor 
rises.  At  length  the  entire  mass  of  liquid  reaches  a 
point  at  which  bubbles  of  its  vapor  may  form  in  parts 
of  its  interior,  and  may  even  acquire  sufficient  vapor 


CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 


53 


pressure  to  come  to  the  surface  of  the  liquid  without 
condensation.     Then  boiling  begins. 

During  the  continuance  of  boiling,  large  amounts  of  heat  may  be  added 
to  the  liquid  without  raising  its  temperature  above  the  boiling-point  for  the 
particular  pressure  prevailing.  Such  heat  is  called  latent  heat  of  vapor- 
ization. It  is  definite  in  amount.  It  is  believed  to  do  some  kind  of 


FIG.  28.  —  Machine  for  producing  artificial  ice  by  the  rapid  evaporation  of  liquefied 
ammonia  gas  (NH3). 


internal  work  on  the  molecules  of  liquid  in  changing  their  structure  or 
positions. 


A  marked  concealment  of  heat  takes  place  when  water  at 


100°  C.  is 

changed  into  steam  of  100°  C.  In  this  case  also  an  amount  of  heat,  equal 
to  that  made  latent,  reappears  when  the  reverse  change  of  state  takes 
place.  That  is,  one  kilogramme  of  water  of  100°  C.  in  changing  to  vapor 
of  100°  C.  absorbs  536  units  of  heat  (without  any  rise  of  temperature). 
Again,  one  kilogramme  of  steam  at  ico°  C.  upon  condensing  to  one  kilo- 
gramme of  water  at  100°  C.  evolves  536  units  of  heat. 


54 


CHANGES    INCIDENT   TO    ADDITION    OF    HEAT. 


In  general,  a  definite  amount  of  heat  becomes  latent  when  any  liquid 
changes  to  the  gaseous  form,  and  it  reappears  when  the  gas  changes  back 
again  to  the  liquid  form. 


FIG.  29.  —  Method  of  employing  currents  of  cold  brine  in  circulating  tubes  for  the 
purpose  of  freezing  the  surface  of  an  artificial  lake.  The  brine  is  cooled  below  the 
freezing-point  of  water  by  an  ammonia  ice  machine. 


Experimental  Demonstration  of  Absorption  of  Heat  during 
Vaporization. — That  liquids  do  in  fact  absorb  heat  in  vaporization  is 
easily  proved  in  case  of  such  liquids  as  water,  petroleum  naphtha,  ethyl 
ether,  etc. 

Other  examples  are  found  in  the  evolution  of  a  gas  from  its  dissolving 
liquid,  as  when  ammonia  gas  escapes  from  a  concentrated  water  solution 
of  ammonia. 

Spontaneous  Evaporation.  —  Certain  substances  evap- 
orate very  readily  when  exposed  in  open  vessels  to  the 
ordinary  temperature  of  the  atmosphere.  In  other 


56          CHANGES    INCIDENT    TO    ADDITION    OF    HEAT. 

words,  they  have  low  boiling-points.  Ethyl  ether  (also 
called  sulphuric  ether)  is  a  good  example  of  a  liquid 
of  this  sort.  Of  course  in  this,  as  in  all  cases,  heat  is 
rendered  latent.  Heat  is  absorbed  from  neighboring 
objects,  and,  as  the  evaporation  proceeds  rapidly,  rapid 
absorption  of  heat  is  associated  with  it.  The  ordinary 
statement  is  that  such  evaporation  precedes  cooling. 


FIG.  31.  — Three  forms  of  receivers  for  collecting  sulphur  dioxide  changed  from  the 
gaseous  form  to  the  liquid  by  cold  and  pressure.  The  second  and  third  forms  are  so  con- 
structed that  a  small  amount  of  liquid  may  be  transferred  from  one  bulb  to  another,  and 
then  may  be  withdrawn  for  experiment. 


Ice  Machines. — The  ice  machines  in  common  use  depend  upon  the 
principles  already  stated.  The  substance  oftenest  used  is  ammonia  gas. 
In  one  part  of  the  machine  ammonia  gas  is  condensed  to  a  liquid  by  the 
aid  of  pressure  and  a  considerable  amount  of  cool  water  circulating  about 
the  vessel  in  which  the  ammonia  gas  is  confined. 

Next  the  liquid  ammonia  is  transferred  to  another  portion  of  the  appa- 
ratus. Here  its  vessel  is  in  indirect  contact  with  the  water  to  be  frozen. 
Upon  opening  a  stopcock,  the  liquid  ammonia  rushes  into  vapor,  instantly 
absorbs  an  immense  amount  of  heat  from  the  water,  and  thereupon  solidi- 
ties it. 


CHANGES    INCIDENT    TO    WITHDRAWAL    OF    HEAT.        5/ 

CHANGES  INCIDENT  TO  WITHDRAWAL  OF  HEAT. 

WITHDRAWAL   OF   HEAT   FROM   A   GAS. 

When  heat  is  withdrawn  from  a  gas  or  vapor,  the  sub- 
stance manifests  various  phenomena,  similar  to  those 
already  referred  to  under  the  head  of  addition  of  heat, 
only,  of  course,  in  the  reverse  order. 

They  are,  among  others,  the  following  :  (i)  Cessation 
of  the  evolution  of  light ;  (2)  recombination  (in  some 
cases  of  dissociation)  ;  (3)  fall  of  temperature  ;  (4)  dim- 
inution of  volume ;  (5)  change  to  the  liquid  form ; 
(6)  evolution  of  latent  heat. 

If  the  liquid  formed  is  subjected  to  further  withdrawal 
of  heat,  other  changes  follow.  (See  p.  59.) 

Liquefaction  of  Gases.  Latent  Heat.  —  In  accordance 
with  the  general  principles  already  enunciated,  it  may 
be  expected  that  liquefaction  of  gases  should  be  attended 
with  evolution  of  heat.  This  is  indeed  the  case.  In 
changing  from  the  state  of  gas  to  the  state  of  liquid, 
heat  is  given  out.  The  heat  is  absorbed  by  any  suitable 
body.  If  the  body  is  cold,  it  becomes  warm  thereby ; 
if  the  body  is  warm,  it  becomes  warmer  still. 

These  two  statements,  simple  as  they  appear,  are  worthy  of  further  ex- 
amination. A  cold  body  condenses  vapors,  the  cold  body  thereby  becom- 
ing slightly  warmer,  of  course.  Almost  the  only  commonly-known  case 
of  a  moderately  warm  body  condensing  vapor  is  where  some  warm  body 
condenses  steam;  i.e.  water-vapor.  If  steam  falls  upon  a  person's  hand, 
the  steam  condenses  to  drops  of  water.  At  the  same  time  latent  heat  is 
evolved,  and  the  hand  may  be  severely  burned.  Again,  in  steam-heating 
appliances  the  steam,  condensing  in  the  suitable  pipes,  evolves  its  latent 
heat  by  the  act  of  condensation,  and  thus  the  air  of  the  building  may  be 
warmed. 


58       CHANGES    INCIDENT    TO    WITHDRAWAL    OF    HEAT. 

Influence  of  Pressure.  —  Of  course  the  liquefaction  of  gases  is 
easier  under  increase  of  pressure;  but,  after  all, pressure  is  only  a  second- 
ary consideration. 

The  English  scientist,  Thomas  Andrews,  discovered  that  however  great 
the  mechanical  pressure  upon  a  gas,  this  pressure  cannot  reduce  the  gas 


FIG.  32.  —  Deleuil's  apparatus  for  the  liquefaction  of  carbon  dioxide  gas.  The  gas  is 
generated  in  the  right-hand  vessel.  The  inner  tube,  containing  sulphuric  acid,  is  broken 
by  forcing  the  tube  against  the  pin  at  the  bottom  of  the  receiver.  The  acid  acts  upon 
carbonate  of  soda,  liberating  carbon  dioxide  gas.  The  latter  passes  into  the  left-hand  re- 
ceiver, and  is  there  condensed  into  the  liquid  form  by  influence  of  the  great  pressure  of  gas. 


to  the  form  of  liquid,  except  when  tJie  gas  is  at  or  below  a  certain  point  of 
temperature.  This  point  differs  for  different  gases.  It  is  called  in  each 
case  the  critical  point. 


CHANGES    INCIDENT   TO    WITHDRAWAL    OF    HEAT.       59 

The  same  facts  may  be  stated,  in  a  sort  of  reversed  form,  as  follows  : 
for  each  liquid  there  is  a  point  of  temperature  at  which  this  liquid  will 
assume  the  gaseous  form,  no  matter  how  great  the  mechanical  pressure 
then  opposing  it.  This  is  another  definition  of  the  critical  point. 


WITHDRAWAL   OF   HEAT   FROM   A   LIQUID. 

c  When  heat  is  withdrawn  from 

a  liquid,  a  series  of  effects  are 
manifested  as  already  indicated, 
n  They  are  in  the  reversed  order  of 
those  noted  when  heat  is  added 
to  a  solid. 


FIG.  33. — Tube  showing  liquid 
ammonia  (NH3)  at  A.  Part  of 
the  ammonia  gas  in  the  tube  is  liq- 
uefied by  great  pressure  and  cold. 


If  the  withdrawal  of  heat  continues,  the 
liquid  begins  to  fall  in  temperature  accord- 
ing to  its  specific  heat.  In  due  time  the  liquid  may  change  to  a  solid. 
For  the  moment  no  fall  of  temperature 
takes  place,  but  the  heat  withdrawn  from 
the  liquid  is  supplied  by  the  liberation  of 
latent  heat.  As  soon  as  solidification 
has  become  complete  the  evolution  of 
latent  heat  ceases,  and  the  solid  begins 
to  fall  in  temperature  in  accordance  with 
its  specific  heat.  At  the  same  time,  of 
course,  it  contracts  in  volume.  The  gen- 
eral statement  of  the  series  of  phenom- 
ena associated  with  withdrawal  of  heat 
is  now  complete. 


Solidification    of    Homogene- 


FIG.  34. — Disposition  of  apparatus 
for  liquefying  ammonia  gas.  In  the 
heated  water-bath,  solution  of  ammo- 
nia gas  in  water  is  decomposed.  The 


ous    Liquids.  —  A    liquid    con-     gas  passes  into.the  other  branch  of 

the  tube,  which  is  surrounded  by  ice. 
Of    a    Single    elementary       Under  the  influence  of  the  cold,  and 


substance,  or  a  liquid  consist-     the  great  pressue  of  th   a°nia 


fk  . 

gas,  a  part  of  the  latter  is  liquefied. 


ing  of  a  single  compound  sub- 

stance, may  be  called,  in  a  general  way,  homogeneous. 

(See  p.    17.)      When   such   liquids   solidify,   they  may, 


6O       CHANGES    INCIDENT    TO    WITHDRAWAL    OF    HEAT. 

under  certain  conditions,  crystallize  ;  under  others,  not. 
The  size  of  the  crystals,  and  in  some  cases  the  partic- 
ular form,  depends  upon  the  circumstances  under  which 
the  solidification  has  taken  place. 

Solidification  of  Mixed  Liquids.  —  There  are  many 
cases  known  in  which  two  distinct  solids,  liquefied  by 
natural  or  artificial  high  temperature,  are  capable  of 


FIG.  35.  —  Apparatus  for  liquefying  sulphur  dioxide  gas.  The  gas  generated  in  A  is 
purified  in  the  wash-bottle  b  and  the  drying-tube  c.  It  then  condenses  to  a  liquid  in  the 
flask  d,  which  is  surrounded  by  a  freezing  mixture. 


very  complete  liquid  interdiffusion.  They  thus  form 
what  maybe  regarded  as  a  single  liquid  —  so  long  as 
the  temperature  does  not  fall  the  constituents  do  not 
separate  from  each  other.  Of  course,  however,  a  suffi- 
cient withdrawal  of  heat  produces  solidification.  It  has 
been  observed,  further,  that  when  the  cooling  proceeds 
very  gradually,  a  series  of  different  substances  separate 
progressively  from  the  liquid.  The  usual  course  is  as 
follows  :  first,  one  of  the  component  solids  separates  ; 


62       CHANGES    INCIDENT    TO    WITHDRAWAL    OF    HEAT. 

next,  some  well-defined  compound  of  the  components 
crystallizes  out ;  sometimes  the  operation  terminates  by 


FIG.  37.  —  Part  of  Cailletet's  apparatus,  showing  its  position  when  the  tube  P 
is  filling  with  the  gas  to  be  experimented  upon. 


the  solidification  of  a  nearly  pure  mass  of  the  other 
component. 


CHAPTER   VII. 
CERTAIN  GENERAL  LAWS  OF  MATTER. 

SIGNIFICANCE   OF  THE   GASEOUS   CONDITION. 

DALTON'S  atomic  theory,  as  now  received,  may  be 
said  to  have  its  basis  in  certain  fundamental  laws  of 
matter.  The  following  arc  some  of  the  most  important 
of  them  :  — 

Boyle's  or  Mariotte's  Law  of  the  Pressure  of  Gases :  — 

% 

The  volume  of  any  gas,  when  confined  under  a  constant 
and  relatively  high  temperature^  varies  inversely  as  the 
pressure  to  which  it  is  exposed. 

The  law  is  easy  of  comprehension. 

It  states  certain  facts  that  are  capable  of  direct  experi- 
mental demonstration. 

It  is  of  wide  application,  for  it  applies  to  any.  gas 
whatsoever. 

It  has  but  one  limitation.  That  is,  a  high  tempera- 
ture is  necessary.  What,  then,  is  high  temperature  ? 
Evidently  the  term  is  relative,  and  its  application  must 
be  interpreted  according  to  the  special  properties  of  the 
gas  in  question. 

The  guide  to  this  interpretation  must  be  found  in  what  is  called  the 
critical  point  for  a  gas.  (See  p.  58.) 

When  a  gas  is  subjected  to  varying  pressure  at  temperatures  far  above 
its  critical  point,  it  conforms  closely  to  the  law  as  stated. 

63 


64 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


At  temperatures  below  its  critical  point  a  gas  is  ready 
to  change  to  a  liquid  by  a  slight  increase  of  pressure. 
But  this  change  brings  the  substance  to  a  condition  in 


FIG.  38.  —  Robert  Boyle.     Born  in  1626;  died  in  1691. 

which  it  obeys  the  laws  of  liquids,  and  not  laws  like  that 
of  Boyle  and  Mariotte,  which  relates  to  gases  exclusively. 
At  temperatures  slightly  above  the  critical  point  a  gas 
is  found  to  manifest  the  beginnings  of  the  influences  of 
the  laws  of  liquids,  and  so  it  does  not  conform  closely 
to  the  laws  of  gases. 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


To  these  explanations  of  the  law  the  following  state- 
ment may  be  added  :  — 

If  a  given  volume  of  any  gas  whatever  is 
subjected  to  the  single  change  of  having  the 
external  pressure  upon  it  doubled,  the  volume 
of  the  gas  is  thereby  reduced  to  one-half ;  and 
the  converse  is  likewise  true  —  that  is,  if  any 
gas  is  subjected  to  the  single  change  of  having 
the  external  pressure  upon  it  reduced  to  one- 
half,  then  its  volume  thereby  becomes  double. 

But  a  given  gas  strictly  conforms  to  this  law 
only  when  it  is  at  temperatures  far  above  that 
of  its  liquefying-point,  wherever  that  may  be. 

Charles's  Law  of  Expan- 
sion of  Gases  by  Heat :  — 

The  volume  of  a  given 
portion  of  any  gas,  under 
a  constant  pressure,  varies 
directly  as  the  temperature 
expressed  in  absolute  centi- 
grade degrees. 

Charles's  law  is  a  formal 
statement  of  facts  that  are 
capable  of  direct  demon- 
stration ;  it  is  one  that  is 
much  used  in  the  study  of 
gases. 

The  ordinary  centigrade 

thermometer  reads  o°  at  the  temperature  of  freezing 
water,  and  it  reads  100°  at  the  temperature  of  boiling 
water. 


FIG.  39.  —  Apparatus  for  showing  that 
different  gases  in  the  tubes  /  and  G  when 
subjected  to  the  same  amount  of  pressure, 
under  condition  of  constant  temperature, 
suffer  the  same  amount  of  reduction  of 
volume. 


FIG.  40. — Apparatus  for  demonstrating  the  truth  of  the  law  of  Charles.  A  portion 
of  gas  from  the  globe  A  may  be  transferred  to  the  tube  BC.  Here  it  may  be  subjected 
to  varying  temperatures,  from  that  of  freezing  water  to  that  of  boiling  water.  The 
change  of  volume  of  the  gas  may  be  read  off  either  from  the  graduations  on  the  tube  BC 
or  else  by  use  of  a  cathetometer.  At  the  same  time  the  gas  may  be  subjected  to  a  given 
pressure  by  adding  mercury  to  the  tube  E,  or  by  withdrawing  it  from  the  bottom  of  the 
apparatus. 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


The  absolute  centigrade  thermom- 
eter reads  273°  at  the  temperature  of 
freezing  water,  and  it  reads  373°  at 
the  temperature  of  boiling  water. 

Now  the  absolute  centigrade  scale 
is  graded  with  reference  to  the  fact 
that  the  coefficient  of  expansion  of 
gases  by  heat  is  the  same  for  all  of 
them ;  namely,  for  an  increase  of  one 
centigrade  degree  the  expansion  is 
2fj  of  the  volume  the  gas  would 
occupy  at  the  temperature  of  freez- 
ing water. 

Since,  then,  the  absolute  scale  is 
graduated  at  273°  at  the  freezing- 
point  of  water,  the  volume  of  any 
gas  increases  under  the  influence  of 
added  heat  at  the  same  rate  as  the 
numbers  on  the  scale  do. 

Rule.  Add  273°  to  any  ordinary 
centigrade  degree,  and  the  sum  gives 
the  corresponding  absolute  centigrade 
degree. 

Graham's  Two  Laws  of  Gaseous  Dif- 
fusion :  — 

1 .  The  diffusion-rate  of  gases  of  the 
same   density  is   the   same,   whatever 
tJieir  chemical  composition. 

2 .  The  relative  diffusion-rates  of  two 
gases  of  different  densities  are  inversely 
as  the  square  roots  of  these  densities. 


FIG.  41.  —  Apparatus 
for  experimental  proof  of 
the  law  of  Boyle  and  of 
Mariotte.  Upon  pouring 
additional  mercury  into 
the  tube  at  C,  additional 
pressure  is  applied  to  the 
portion  of  gas  at  A  B. 
The  volume  of  gas  is 
thereby  reduced  in  ac- 
cordance with  the  law. 


68 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


These  laws  are  statements  of  facts  capable  of  direct 
demonstration.  When  portions  of  gases  of  different 
densities  are  brought  into  direct  contact,  or  are  sepa- 
rated by  a  porous  partition  only,  the  molecules  of  both 


FIG.  42.  — Jacques  Alexandra  Ce"sar  Charles.     Born  in  1746;  died  in  1823. 
Celebrated  French  physicist. 

gases  are  discovered  to  be  in  active  motion.  Incident- 
ally the  gases  intermingle.  These  properties  of  gases 
are  displayed  by  the  various  forms  of  the  diffusiometer. 
If  a  portion  of  gas  is  confined  within  the  porous  cell 
of  a  diffusiometer,  this  gas  projects  its  molecules  into 


FIG.  43.  —  One  form  of  Torricellian  barometer,  with  its  cathetometer.  In  this  case 
the  barometer  is  EF.  The  apparatus  in  the  figure  is  intended  to  measure  the  pressure 
r>r  mercury  upon  a  portion  of  gas  in  the  tube  AB. 


7<3  CERTAIN    GENERAL    LAWS    OF    MATTER. 

the  passages  of  the  porous  walls,  and  they  pass  through 
and  out.  Simultaneously,  and  in  the  same  manner,  any 
external  gas  projects  its  molecules  inward.  But  the 
rate  of  passage  is  found  to  be  different  if  the  gases  have 
different  densities.  This  is  proved  by  the  motion  of  the 
liquid  in  the  gauge-tube  connecting  with  the  cell  of  the 
diffusiometer.  It  is  observed  that  the  molecules  of  the 


FIG.  44.  —  Graham's  apparatus  for  showing  diffusion  of  hydrogen  gas.  The  tube  A 
contains  hydrogen  gas.  The  top  of  the  tube  is  closed  by  a  porous  wafer.  The  hydrogen 
gas  escapes  so  rapidly  into  the  air  that  the  atmospheric  pressure  upon  the  mercury  in  the 
trough  is  able  to  force  the  mercury  up  into  the  tube  A, 

lighter  gas  always  move  with  greater  rapidity.  A  series 
of  careful  experiments  has  afforded  the  basis  for  the  law 
already  stated. 

The  following  is  an  illustration  of  this  law :  A  given 
bulk  of  oxygen  gas  is  found  by  experiment  to  weigh  six- 
teen times  as  much  as  the  same  volume  of  hydrogen 
gas.  The  density  of  oxygen  is  then  said  to  be  sixteen 
(it  being  customary  to  adopt  hydrogen  gas  as  the 


CERTAIN  GENERAL  LAWS  OF  MATTER.       J\ 

standard  of  density  for  gases).  Now  hydrogen  gas  is 
found  experimentally  to  diffuse  itself  into  oxygen  gas 
four  times  as  rapidly  as  oxygen  gas  diffuses  into  it. 


The  Law   of  Henry   and   of   Dalton,   of  the   Relation   of 
Pressure  to  the   Solubility   of  a  Gas  in   Water :  — 

When  a  given  gas  is  exposed  to  water 
under  a  constant  temperature,  the  volume 
of  the  gas  dissolved  by  the  water  varies 
directly  as  the  pressure  acting  at  the  time. 

The  amount  of  gas  dissolved  by  water 
varies  with  the  nature  of  the  gas.  It 
also  varies  with  the  temperature,  being 
in  general  less  at  higher  temperatures. 
It  also  varies  with  the  pressure  acting. 
This  last  is  the  only  one  of  the  three 
conditions  that  can  be  described  in  the 
form  of  a  law.  By  the  law  as  given,  it 
appears  that  a  gas  resting  on  the  sur- 
face of  water  is  dissolved  by  the  water 
to  a  certain  extent.  If  now  (other  eon- 

FIG.    45.  —  Syphon 

ditions  being  appropriate)  the  pressure,  for  containing  water 
for  example,  is  doubled,  the  amount  of  2^!*  carb°n 
gas  dissolved  will  also  be  doubled.  It 
likewise  follows  that  if  the  pressure  is,  for  example, 
halved,  the  amount  of  gas  dissolved  will  be  halved  ;  in 
other  words,  under  lessened  pressure  gas  dissolved  in 
water  actually  comes  out  of  the  water,  escaping  with 
effervescence.  If  water,  charged  with  gas  under  pres- 
sure, is  allowed  to  flow  from  a  syphon,  the  gas  imme- 
diately leaves  the  water  with  effervescence. 


72  CERTAIN    GENERAL    LAWS    OF    MATTER. 

The  Law  of  Definite  Proportions  :  — 

The  same  compound  always  contains  the  same  atoms, 
united  in  the  same  proportions  by  weight  (and  by  volume 
when  they  are  gaseous],  and  with  the  same  molecular 
arrangement. 

Example:  Pure  water  always  contains  in  each  mole- 
cule only  one  atom  of  oxygen  and  only  two  atoms  of 
hydrogen.  It  always  contains  these  atoms  in  approx- 
imately the  following  proportions,  by  weight :  — 

Hydrogen  ....       2  parts  by  weight. 

Oxygen 16  parts  by  weight. 

18 

It  always  contains  these  atoms  in  the  following  pro- 
portions, by  volume  :  Two  volumes  of  water  vapor  when 
decomposed  yield  approximately  :  — 

Hydrogen two  volumes. 

Oxygen one  volume. 

The  molecular  arrangement  is  believed  to  be  such 
that  the  oxygen  atom  is  somehow  between  the  two  hy- 
drogen atoms,  —  an  arrangement  which  may  be  expressed 
by  the  formula,  H  —  O  —  H. 

This  law  contains  several  statements.  Some  of  them 
are  capable  of  experimental  demonstrations ;  some  of 
them  are  not.  But  the  latter  are  based  upon  a  multi- 
tude of  observed  facts  which  strongly  suggest  their 
truth. 

The  law  as  a  whole  is  implicitly  and  safely  relied  upon 
in  all  chemical  experiments  and  in  the  conduct  of  the 
great  chemical  industries  of  the  world. 


CERTAIN    GENERAL    LAWS    OF    MATTER.  73 

The  Two  Laws  of  Multiple  Proportions :  — 

1.  When  the  elementary  substance  A  chemically  unites 
with  the  elementary  substance  B  in  more  than  one  propor- 
tion by  weight  (and  in  case  of  gaseous  elements,  by  volume 
as  well),  they  form  more  than  one  compound,  and  the 
several  compound  substances  so  produced  possess  well- 
marked  and  distinctly  different  properties. 

2.  The  several  amounts  by  weight  (and  in  case  of  gas- 
eous  elementary  substances,  by  volume  also)  of  B,  that 
may  combine  with  the  same  amount  of  A,  bear  a  very 
simple  relation  to  each  other. 

As  a  general  example  illustrating  this  law,  suppose 
that  the  elementary  substances  A  and  B  unite  in  the 
several  proportions  expressible  by  the  formulas  :  — 

A  Bm,  A  Bn,  A  B0,  A  Bp,  A  Bq.  —  It  is  found  that  the 
five  compounds  formed  are  essentially  different  sub- 
stances. It  is  found  that  several  amounts  of  B  repre- 
sented by  m,  n,  o,  p,  q,  bear  very  simple  ratios  to  each 
other. 

As  a  special  example  the  compounds  of  nitrogen  and 
oxygen  may  be  taken. 

These  two  substances  unite  in  five  different  propor- 
tions by  weight  and  gaseous  volume. 

They  produce  five  distinctly  different  compounds,  each 
having  special  characteristics  of  its  own.  They  are  the 
following :  — 

Nitrogen  Monoxide  (N^O). 

This  is  composed  of  28  parts  nitrogen  by  weight 
and   1 6       "      oxygen      "         " 
44 


74  CERTAIN    GENERAL    LAWS    OF    MATTER. 

Nitrogen  Dioxide  (N2O2  or  NO). 
This  is  composed  of  28  parts  nitrogen  by  weight 
and  32       "     oxygen      "        " 
60 


Nitrogen  Trioxide 
This  is  composed  of  28  parts  nitrogen  by  weight 
and  48       "     oxygen      "        " 
76 


Nitrogen  Tetroxide  (N2O^  or  NO2). 
This  is  composed  of  28  parts  nitrogen  by  weight 
and  64      "      oxygen      "         " 
92 

Nitrogen  Pentoxide  (N2OS). 

This  is  composed  of  28  parts  nitrogen  by  weight 
and  80      "      oxygen      "         " 
108 

Evidently  in  these  five  compounds  the  several  weights 
of  oxygen  combined  with  the  constant  weight  of  nitro- 
gen bear  the  simple  ratios  I  :  2  :  3  :  4  :  5.  (For  further 
discussion  of  these  compounds,  see  pp.  232,  236.) 

NOTE.  It  will  be  observed  that  in  the  five  compounds  just  cited  in  illus- 
tration of  the  laws  under  consideration,  the  amount  of  nitrogen  is  taken  as 
a  basis  of  comparison,  and  its  weight  is  represented  by  the  number  28. 
This  number  has  been  used  because  it  has  been  found,  after  a  great  multi- 
tude of  most  carefully  devised  and  executed  experiments,  that  it  has  some 
special  significance  in  this  case.  //  is  believed  to  represent  twenty-eight 
microcriths  ;  i.e.  the  weight  of  the  amount  of  nitrogen  present  in  one  mole- 
cule of  each  of  the  substances  referred  to. 

In  its  first  stages  quantitative  chemical  analysis  makes  its  statements  in 
percentage  form.  In  the  cases  of  the  nitrogen  compounds  referred  to, 
such  statements  are  given  in  columns  two  and  three  of  the  following 
table:  — 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


75 


1 

2 

3 

4 

5 

PER  CENT  i 

JY  WEIGHT. 

WEIGHT-RATIO. 

VOLUME-RATIO. 

Nitrogen 

Oxygen 

Nitrogen  :  Oxygen 

Nitrogen  :  Oxygen 

Nitrogen  monoxide 

63-7I 

36.29 

=     -57    (0 

i:    £   (I) 

Nitrogen  dioxide 

46.75 

53.25 

:  1.14   (2) 

I  :  I      (2) 

Nitrogen  trioxide 

36.91 

63.09 

:i-7i   (3) 

I'll  (3) 

Nitrogen  tetroxide 

30-5° 

69.50 

:  2.28  (4) 

1:2     (4) 

Nitrogen  pentoxide 

25.98 

74.02 

:  2.85   (5) 

i  =  4  (5) 

If  in  these  several  compounds  the  amount  by  weight  of  nitrogen  in  each 
is  taken  as  a  common  unit  of  comparison,  then  the  amounts  by  weight  of 
oxygen  will  be  .57:  1.14:  1.71  :  2.28:  2.85,  and  these  numbers  appear  at 
once  by  inspection  to  be  to  each  other  as  i  :  2 :  3 :  4 :  5. 

It  is  plain  then  that  the  several  amounts  of  oxygen  in  these  compounds, 
combined  with  the  same  amount  of  nitrogen,  bear  to  each  other  the  very 
simple  ratio  stated  as  I  :  2 :  3 :  4 :  5. 

In  its  most  advanced  stages,  quantitative  chemical  analysis  employs 
for  each  elementary  substance  a  number  representing  its  atomic  or  molec- 
ular weight.  Thus  the  atomic  weight  of  nitrogen  is  believed  to  be  14 
microcriths,  and  its  molecular  weight  28  microcriths.  The  atomic  weight 
of  oxygen  is  believed  to  be  16  microcriths,  and  its  molecular  weight  32 
microcriths. 

Returning  now  to  the  table  given,  it  is  discovered  that  the  volume-ratios 
afforded  by  the  five  compounds  show  yet  more  marked  simplicity.  Not 
only  do  the  several  volumes  of  oxygen  bear  to  each  other  the  simple  ratios 
1:2:3:4:5,  but  they  also  in  each  case  bear  a  very  simple  ratio  to  the 
constant  volume  of  the  other  element,  the  nitrogen.  (See  the  laws  of 
Gay-Lussac.) 

Gay-Lussac's  Three  Laws  of  Combination  of  Gases  :  — 

1.  When  tivo  or  more  gases  combine,  the  volumes  of 
these  gases  bear  very  simple  ratios  to  each  other. 

2.  When  tiuo  or  more  gases  combine  to  form  a  product 
wJiich  can  remain  a  gas,  the  volume  of  the  gas  so  formed 


76  CERTAIN    GENERAL    LAWS    OF    MATTER. 

bears  a  very  simple  ratio  to  the  volume  of  each  of  the  com- 
ponent gases. 

3.    The    weights    of  combining    volumes    of  gaseous 


FIG.  46.  —  Gay-Lussac.     Born  1778;  died  1850. 

elements     bear    very    simple     ratios     to     their    atomic 
weights. 

(In  all  these  cases  it  is  understood  that,  when  under 
comparison,  the  gases  are  at  constant  temperatures  and 
pressures?) 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


77 


Many  illustrations  of  the  signification  of  these  state- 
ments might  be  given.  Two  useful  ones  are  presented 
here. 

First  Illustration  drawn  from   Hydrochloric   Acid    Gas. 

-  The  principal  points  to  be  mentioned  in  connection 
with  the  matter  here  under  consideration  may  be   ex- 


pressed by  the  following  equations  :  — 


H2 

+  '                C12 

One  molecule  of 

One  molecule  of 

Hydrogen, 

Chlorine, 

2 

71 

parts  by  weight. 

parts  by  weight. 

73 


2HC1 

Two  molecules  of 
Hydrochloric  acid, 

73 
parts  by  weight. 

73 


H 

I 
—  H 

I 

Cl- 

-Cl 

H 

1 
-Cl 

I 

H 

I 
-Cl 

I 

I  +  I 

2  71 

parts  by  weight.       parts  by  weight. 


i  +35i 
36* 

parts  by  weight.       parts  by  weight. 


FIRST  FACT.  —  Hydrogen  gas  and  chlorine  gas  chemically  combine. 

SECOND  FACT.  —  As  a  result  they  produce  a  new  gas  called  hydro- 
chloric acid  gas,  having  properties  different  from  those  of  the  components. 

THIRD  FACT.  —  When  hydrogen  and  chlorine  combine,  they  do  so  in 
the  proportions  of  two  volumes  of  chlorine  and  two  volumes  of  hydrogen. 

FOURTH  FACT. — The  resulting  gas  has  the  bulk  of  four  volumes;  in 
other  words,  there  is  neither  permanent  contraction  nor  permanent  expan- 
sion as  a  result  of  the  act  of  union. 

FIFTH  FACT.  —  The  combining  volumes  of  hydrogen  gas  and  chlorine 
gas  have  the  weight-ratio  of  2:  71  =  i  :  35.5.  Now  the  accepted  atomic 
weight  of  hydrogen  is  I,  and  that  of  chlorine  is  35.4. 

Evidently  then  the  facts  stated  sustain  the  statements  of  the  law. 

Second  Illustration   drawn  from    Water   Vapor.  —  The 

following  equations  are  applicable  in  this  case  :  — 


CERTAIN    GENERAL    LAWS    OF    MATTER. 


2H2 

Two  molecules  of 
Hydrogen, 

4 
parts  by  weight. 


One  molecule  of 
Oxygen, 

32 
parts  by  weight. 


2H20 

Two  molecules  of 
Water  vapor, 

36 
parts  by  weight. 


H 

1 
-H 

I 

H 

I 
-H 

I 

I  + 


16  +  16  2  +  16       2  H-  16 


4  32  36 

parts  by  weight.  parts  by  weight.  parts  by  weight. 

FIRST  FACT.  —  Hydrogen  gas  and  oxygen  gas  chemically  unite. 

SECOND  FACT.  —  As  a  result  they  produce  a  new  gas  or  vapor  called 
hydric  oxide  or  water  vapor,  having  properties  different  from  those  of  the 
component  gases. 

THIRD  FACT.  —  When  hydrogen  and  oxygen  combine,  they  do  so  in  the 
proportions  of  four  volumes  of  hydrogen  and  two  volumes  of  oxygen. 

FOURTH  FACT.  —  The  resulting  gas  has  the  bulk  of  four  volumes;  that 
is,  the  same  weight  of  matter  has  been  packed  into  smaller  space  by  the 
influence  of  the  chemical  union  which  has  taken  place.  Thus  there  has 
been  a  permanent  reduction  from  a  total  of  six  volumes  to  a  total  of  four 
volumes.  The  ratio  of  6 :  4  =  3  :  2,  and  is  a  very  simple  one. 

FIFTH  FACT.  —  The  combining  volumes  of  hydrogen  gas  and  oxygen 
gas  have  the  weight-ratio  of  4:  32  =  2:  16.  Now  the  accepted  atomic 
weight  of  hydrogen  is  i,  and  that  of  oxygen  is  16. 

Evidently,  then,  the  facts  stated  under  this  illustration  sustain  the  laws 
as  given. 

Avogadro's  and  Ampere's  Hypothesis  of  the  Size  of 
Gaseous  Molecules  :  — 

Equal  volumes  of  all  substances,  when  in  tJie  gaseous 
state,  and  under  like  conditions  of  temperature  and  pres- 
sure, contain  the  same  number  of  molecules. 


CERTAIN  GENERAL  LAWS  OF  MATTER.       79 

Evidently  the  hypothesis  declares  that  if,  under  cer- 
tain conditions,  one  cubic  foot  of  oxygen  gas  contains  n 
molecules  of  oxygen,  then  under  the  same  conditions 
one  cubic  foot  of  nitrogen  gas,  one  cubic  foot  of  hydro- 
gen gas,  one  cubic  foot  of  compound  gases,  as  carbon 
dioxide  (CO2),  ammonia  gas  (NH3),  each  contain  n  mol- 
ecules of  their  respective  substances. 

This  hypothesis  seems  to  follow  directly  from  the 
laws  of  Boyle  and  of  Charles.  For  if  the  material 
groups  we  call  molecules  exist  at  all,  and  if  expansion 
and  contraction  of  gases  are  in  fact  due  to  the  mov- 
ing apart  or  the  moving  together  of  these  material 
groups,  then  the  observed  exact  correspondence  of  the 
laws  of  such  expansion  and  contraction,  even  in  differ- 
ent substances,  points  conclusively  to  the  existence 
of  equal  numbers  of  molecules  in  equal  bulks  of 
gases. 

The  hypothesis  of  Ampere  is  not  the  expression  of  a 
distinct  and  easily  verified  fact.  It  is  rather  the  most 
reasonable  explanation  of  a  series  of  facts  which  cannot 
well  be  correlated  without  it. 

As  an  example  of  its  bearing,  the  following  chemical  illustration  may  be 
given :  — 

FIRST.  Ammonia  gas  (NH3)  and  hydrochloric  acid  gas  (HC1)  may 
be  proved,  by  processes  independent  of  this  hypothesis,  to  have  the  for- 
mulas here  assigned  them. 

SECOND.  They  are  found  by  analysis  to  have  the  respective  molecular 
weights  17  and  36.4. 

THIRD.  It  is  found  experimentally  that  they  always  combine  in  the 
proportion  of  1 7  parts  by  weight  of  ammonia  gas  to  36.4  parts  by  weight 
of  hydrochloric  acid  gas.  Hence  they  unite  molecule  for  molecule. 

FOURTH.       It  is  found  that  they  unite  in  equal  volumes. 

Whence  these  equal  volumes  appear  to  contain  equal  numbers  of 
molecules. 


8O  CERTAIN    GENERAL    LAWS    OF    MATTER. 

The  Relation  of  Diffusion  of  Gases  to  Ampere's  Law.  —  That 

the  facts  relating  to  diffusion  of  gases  afford  a  remarkable  confirmation  of 
the  law  of  Avogadro  and  of  Ampere,  may  be  made  apparent  from  the  fol- 
lowing example  and  explanations  :  — 

Suppose  a  rubber  balloon  containing  hydrogen  gas  and  exposed  to  the 
air.  The  hydrogen  gas  is  subject  to  two  pressures  from  without,  —  the  con- 
tractile force  of  the  rubber,  and  the  weight  of  the  atmosphere.  Why  then 
is  it  not  reduced  to  yet  smaller  bulk  ?  Because  of  a  resistance  to  pressure 
that  it  possesses  in  common  with  other  forms  of  matter.  In  gases  this 
resistance  is  called  tension,  or  elasticity,  or  expansive  power.  But  the 
facts  of  diffusion  prove  that  all  gaseous  molecules  are  in  particularly  active 
motion,  though  with  different  rates.  We  are  thence  led  to  believe  that  the 
tension  of  the  hydrogen  in  the  balloon  referred  to  is  due  to  the  impact  of 
the  moving  hydrogen  molecules;  that  is,  to  their  outward  blows  against  the 
enclosing  walls  of  the  balloon. 

Again,  suppose  a  balloon  containing  oxygen,  but  otherwise  in  every 
way  similar  to  that  containing  hydrogen,  just  discussed.  The  portion 
of  oxygen  gas  will  have  a  weight  sixteen  times  that  of  the  hydrogen  gas. 
The  oxygen  gas  possesses  tension,  and  this  is  due  to  the  outward  impact  of 
the  oxygen  molecules  against  the  walls  of  its  containing  vessel. 

Now  it  is  to  be  noted  that  in  this  second  case  the  impact  is  equal  to  the 
impact  of  the  hydrogen  molecules  in  the  other  case;  for  in  both  cases  the 
same  external  pressures  are  overcome. 

The  laws  of  mechanics  show  that  the  impact  of  any  moving  body  may 
be  expressed  as  equal  to  one-half  its  mass  multiplied  by  the  square  of  its 
velocity.  Then  the  impact  of  a  moving  molecule  of  hydrogen  and  of  oxy- 
gen may  be  expressed  respectively  as  follows :  — 

i  (impact  of  molecule  of  hydrogen)  =  J  mv2; 
i' (impact  of  molecule  of  hydrogen)  —  £  m'v1'2. 

But  it  has  been  shown  already  that  in  the  case  of  equal  volumes  of  the 
two  gases, 


whence         mv2=m'v'2. 

Taking  the  velocity  of  the  oxygen  molecule  (z/)  as  the  unit  of  compari- 
son of  velocity  and  calling  it  I,  and  substituting  for  ///  and  m'  the  ratios  of 
their  respective  molecular  weights,  i  and  16,  we  obtain, 

»a=  16, 

v=   4. 


CERTAIN    GENERAL    LAWS    OF    MATTER.  8 1 

This  result  means  that  when  the  velocity  of  the  oxygen  molecule  is 
called  i,  the  velocity  of  the  hydrogen  molecule  is  4.  Now  this  is  in  fact 
the  rate  of  motion  of  the  molecules  as  proved  by  Graham.  And  the  result 
obtained  from  the  course  of  reasoning  here  pursued  has  involved  but  one 
supposition ;  namely,  that  the  two  equal  balloons,  or  in  fact  any  two  equal 
volumes  of  the  gases,  under  like  conditions  of  temperature  and  pressure, 
contain  equal  numbers  of  molecules. 

The  correct  result  attained  contributes  materially  to  place  the  hypothe- 
sis of  Avogadro  upon  a  mathematical  foundation. 


CHAPTER   VIII. 

CERTAIN  FORMS  OF  ENERGY  CLOSELY  CONNECTED 
WITH  CHEMICAL  CHANGES. 

HEAT    AND   ELECTRICITY. 

IT  has  been  remarked  that  "  a  chemical  operation  pre- 
sents two  aspects  to  the  investigator ;  it  involves  a 
change  in  the  form  or  distribution  of  matter  and  a 
change  in  the  form  or  distribution  of  energy." 

Two  forms  of  energy  are  especially  involved  in  chem- 
ical changes  :  they  are  heat  and  electricity. 

These  subjects  belong  in  a  certain  sense  to  the  de- 
partment of  physics,  yet  by  their  sources,  uses,  and 
effects,  they  are  so  closely  connected  with  chemistry 
that  they  admit  of  brief  discussion  here. 

HEAT. 

The  invisible  agency  by  whose  transfer  sensations  of 
warmth  and  cold  are  produced,  is  itself  called  heat. 
Two  kinds  of  heat  may  be  distinguished  :  — 

1.  Absorbed  heat  is  that  which  resides  in  a  hot  body, 
often   remaining  in   it  for  a   considerable  time.     It   is 
transferred  to  another  body,  mainly  by  contact. 

2.  Radiant  heat  is  heat  in  the  act   of  passing  with 
great  velocity  (about  190,000  miles  per  second)  through 

space,  whether  vacuous  or  otherwise  ;   radiant  heat  may 
82 


CERTAIN  FORMS  OF  ENERGY.  83 

be  either  dark  heat  or  luminous  heat  (the  latter  form 
being  commonly  known  as  light). 

Theories  of  the  Nature  of  Heat.  —  The  dynamical  the- 
ory of  heat,  and  that  now  generally  accepted,  supposes 
that  all  matter  as  well  as  all  space  is  pervaded  by  an 
extremely  delicate  and  elastic  medium  called  the  ether. 
This  theory  regards  (i)  absorbed  heat  as  a  vibration  of 
the  molecules  of  matter ;  (2)  radiant  heat  as  an  undula- 
tory  movement  in  the  ether. 

Heat  as  Motion.  —  That  heat  is  not  a  form  of  matter 
appears  to  be  shown  by  a  variety  of  facts.  For  exam- 
ple, neither  does  addition  of  heat  to  a  body  increase  its 
weight,  nor  does  loss  of  heat  by  a  body  diminish  that 
weight. 

The  quantity  of  heat  in  a  given  system  is  capable 
of  indefinite  increase  ;  again,  it  can  be  destroyed  as 
material  substances  cannot. 

That  heat  is  a  form  of  energy  —  in  other  words,  of 
motion  —  appears  to  be  suggested  by  a  multitude  of 
phenomena.  Of  these  a  few  may  be  mentioned. 

FIRST.  The  general  quantitative  relations  between  heat  and  mass- 
motion  are  very  simple.  A  given  amount  of  mechanical  motion  may  be 
changed  into  a  certain  definite  amount  of  heat  and  no  more;  and  on  the 
other  hand,  a  given  amount  of  heat  is  capable  of  generating  only  a  certain 
fixed  amount  of  mass-motion. 

Some  of  the  contrivances  ordinarily  used  to  effect  such  interchange  are 
mperfect  and  involve  large  losses  during  the  transformations;  these,  how- 
ever, are  not  losses  of  the  total  amount  of  energy,  but  only  of  that  partic- 
ular form  of  it  which  the  appliance  or  machine  may  be  intended  to  afford. 
Hence  the  strength  of  the  argument  is  not  impaired. 

SECOND.  The  sources  of  heat  are  well  explained  by  this  view.  They 
are  friction,  percussion,  chemical  action,  the  sun.  (The  internal  heat  of 
the  earth  need  not  be  discussed  here.) 


84 


CERTAIN  FORMS  OF  ENERGY. 


Friction  and  percussion  involve  a  diminution  of  mass-motion  —  or  its 
entire  quenching.  But  these  have  not  been  destroyed ;  they  appear  to 
have  been  merely  transformed  into  minute  molecular  motions. 

Chemical  action  evolves  heat  when  certain  substances  combine.  The 
true  source  of  this  heat  appears  to  be  that  molecular  percussion  or  atomic 
bombardment  which  is  sustained  when  a  myriad  of  atoms  of  one  kind 
clash  into  combination  with  a  myriad  of  another  kind. 

The  sun  gives  out  an  enormous  amount 
of  heat.  Only  a  minute  fractional  part  of 
it  is  received  by  this  earth.  (But  this  is 
a  large  amount  as  compared  with  man's 
ordinary  means  of  producing  energy.)  But 
this  tremendous  and  continual  transfer  does 
not  appear  to  diminish  the  weight  of  the 
giver  nor  to  increase  that  of  the  receiver. 

Again,  it  appears  more  rational  to  be- 
lieve that  the  incredible  velocity  of  radiant 
heat  is  associated  with  a  progressive  flow 
of  energy  rather  than  with  an  actual  trans- 
portation of  matter. 

THIRD.  — The  effects  of  heat  are  best 
explained  by  this  view.  The  principal  of 
these  effects  are  the  following :  (a)  tem- 
perature, (£)  expansion  and  contraction, 
(c)  change  of  state,  (d)  work,  (<?)  light, 
(/)  chemical  combination  and  decompo- 
sition, (£•)  electricity. 

FIG.  47.  -  Apparatus  for  gradu-         O)  Temperature.  —When  a  portion 

ating  a  thermometer  at  the  freezing-    of  matter    gains    or   loses    heat,   the    effect 

point  of  water.  easiest  and  oftenest  noticed  is  rise  or  fall 

of  temperature.      (But  it  is  well   known 

that  in  some  cases  this  effect  is  altogether  wanting.    See  latent  heat,  p.  47.) 

What,  then,  is  signified  by  the  temperature  of  a  body?  Evidently  its 
state  of  sensible  thermal  equilibrium  or  want  of  equilibrium  as  compared 
with  some  other  body. 

Temperature,  then,  is  relative.  It  expresses  the  condition  of  a  body  in 
answer  to  the  questions,  does  this  body  give  heat  to  our  persons?  or,  does 
it  take  heat  from  them?  does  this  body  give  or  withdraw  heat  from  some 
other  certain  neighboring  body  with  which  it  may  be  placed  in  contact? 


CERTAIN  FORMS  OF  ENERGY. 


To  one  of  these  questions  our  own  nervous  systems  give  a  more  or 
less  distinct  answer;  to  another  we  get  an  answer  by  observing  some 
convenient  secondary  effect,  such  as  the  expansion  of  matter  in  some 
thermoscope. 

The  terms  warm  and  cold  mean,  then,  the  transfer  of  a  certain  force  in 
one  direction  or  another  toward  a  given  body  or  from  it.  And  it  is  sup- 
posed that  this  transfer  results  in  an  increase  or  a  decrease  of  molecular 
motion. 

But  it  is  supposed  that  no  known  body  exists  in  the  condition  of  pos- 
sessing absolutely  no  heat;   that  is,  no  molecular  motion.     Such  a  condi- 
tion might  be  characterized  as  at  a  tem- 
perature of  absolute  zero  (this  temperature 
in  assumed  to  be  minus  273  degrees  ordi- 
nary centigrade).     Finally,  it  can  easily  be 
shown  that  temperature,  whether  judged 
by  our  sensations  or  by  thermoscopes,  is 
no  certain  index  of  the  real  amount  of  heat 
possessed  by  a  body. 

(£)  Expansion.  —  With  few  excep- 
tions all  bodies,  whether  solid,  liquid,  or 
gaseous,  expand  with  addition  of  heat  and 
contract  upon  withdrawal  of  it.  In  these 
changes  the  heat  certainly  produces  a  mass- 
motion.  It  is  observed  in  the  alteration  of 
the  bulk  of  the  whole  mass  of  the  body 
acted  on,  but  apparently  due  to  an  increase 
or  diminution  of  the  motion  of  the  mole- 
cules. 


FIG.  48.  —  Steam  box  for  use  in 
graduating  a  thermometer  at  100°  C. 


(V)  Change  of  State. —  When  the  molecular  motion  just  referred  to 
becomes  so  great  as  to  carry  the  molecules  of  a  solid  or  of  a  liquid  beyond 
the  range  of  those  cohesive  forces  which  characterize  its  previous  state, 
it  changes  to  the  next  less  dense  form  of  matter.  In  other  words,  addition 
of  heat  may  in  most  cases  change  a  solid  to  a  liquid  and  a  liquid  to  a  gas. 
Again,  withdrawal  of  heat  may  so  restrict  the  molecular  motion  as  to  bring 
the  molecules  of  a  gas  near  enough  together  to  make  them  subject  to  such 
cohesive  forces  as  bind  them  into  a  liquid,  or  even  to  a  solid. 

There  are  some  substances  that  have  not  yet  been  made  to  undergo 
change  of  state.  Carbon,  for  example,  has  not  yet  been  changed  from  the 
solid  to  the  liquid  form,  —  much  less  to  the  gaseous.  But  it  can  hardly  be 


86 


CERTAIN  FORMS  OF  ENERGY. 


called  an  exception  to  the  general  statement.  For  in  the  first  place  the 
number  of  such  substances  is  steadily  decreasing;  witness  the  recent  lique- 
faction of  the  so-called  permanent  gases,  oxygen,  hydrogen,  nitrogen,  and 
the  like.  In  the  second  place,  all  experience  shows  that  whenever  refrac- 
tory solids  are  liquefied  and  vaporized,  it  is  in  consequence  of  addition  of 
heat;  and  whenever  liquids  not  previously  frozen  are  solidified,  it  is  in 

consequence  of  withdrawal  of 
heat.     (See  p.  58.) 

(</)  Chemical  Combi- 
nation and  Decomposi- 
tion. —  These  subjects  are 
important,  but  they  are  ex- 
plained later. 

0)  Light.  — For  the  pur- 
poses of  this  book  this  sub- 
ject may  be  presented  in 
connection  with  spectrum  an- 
alysis. 

SPECTRUM   ANALYSIS. 

Spectrum  analysis  is  much 
used  in  chemistry.  It  de- 
pends upon  the  following 
facts : — 

1.  Highly    heated     solids 
give  out  white    light;    i,e.  a 
mixture   of  rays   of  light   of 
various  colors. 

2.  Highly  heated  gases  or 

vapors  give  out  light  of  a  characteristic  color;    i.e.  either  monochromatic 
light    (composed   of  rays   of  a   single    quality),    or    polychromatic    light 
(composed  of  rays  of  a  character  such  that  they  cannot  make  up  white 
light). 

3.  A  ray  of  light  when  it  passes  through  a  prism  is  bent  out  of  its 
course,  i.e.  suffers  refraction;  while  a  bundle  of  rays  passing  through  a 
prism  suffers  dispersion;  that  is,  the  different  rays  of  which  the  bundle  is 
composed  are  bent  out  of  course  in  different  degrees. 


FIG.  49.  —  Glass  prism  in  a  convenient  mounting 
for  use  in  experiments  upon  the  refraction  of  light. 


CERTAIN  FORMS  OF  ENERGY.  87 

The  Spectrum.  —  The  term  spectmm  is  applied  to  that  peculiar  pic- 
ture which  appears  when  a  narrow  beam  of  light,  after  passing  through  a 
prism,  is  allowed  to  fall  upon  a  screen. 

Thus  white  light  when  passed  through  a  prism  gives  a  spectrum  that 
contains  an  enormous  number  of  rays.  These  rays  are  of  all  the  different 
colors  and  shades  of  the  rainbow,  and  they  appear  to  be  the  same  what- 
ever the  chemical  constitution  of  the  body  giving  out  the  white  light.  The 
spectrum  is  called  a  continuous  one. 

But  colored  light  produces  a  discontinuous  spectrum.     Moreover,  the 


Fig.  50.  —  Refraction  of  light  by  water.  A  beam  of  light  coming  from  the  rod  at  the 
bottom  of  the  water  is  bent  out  of  course  so  that  when  it  comes  to  the  eye  it  appears  to 
arrive  from  a  different  point,  and  thus  the  rod,  in  fact  straight,  appears  to  be  bent. 


character  of  such  a  spectrum  varies  with  the  chemical  substance  produc- 
ing it. 

The  orange  light  emitted  by  highly  heated  sodium  appears  upon  ordi- 
nary examination  to  be  monochromatic.  In  a  certain  sense  it  is  so.  But 
critical  investigation  shows  that  its  well-known  color  is  made  up  of  several 
minutely  different  shades. 

The  Spectroscope.  —  The  following  are  a  few  of  the  fundamental 
principles  upon  which  the  use  of  the  spectroscope  depends :  — 

I.  A  spectroscope  is  a  contrivance  for  examining  light.  In  its  simplest 
form  it  consists  of  three  parts,  —  a  narrow  opening  to  admit  the  beam  of 
light  to  be  tested;  a  prism  or  series  of  prisms  to  disperse  the  rays  of  light; 
a  small  telescope  to  bring  the  spectrum  to  the  eye. 


88  CERTAIN  FORMS  OF  ENERGY. 

2.  A  beam  of  light  when  passed  through  a  suitable  spectroscope  yields 
a  spectrum  showing  of  what  ray  or  rays  the  beam  consists. 

3.  A  beam  of  light  may  be  tested  just  as  it  comes  from  its  source;   thus 
any  substance  may  by  some  method  be  so  heated  as  to  become  luminous, 
—  that  is,  to  give  out  light,  —  and  such  light  may  be  passed  through  the 
spectroscope. 

One  method  of  heating  is  by  an  ordinary  Bunsen  lamp. 

A  second  method  applicable  to  solids  and  liquids  is  to  cause  some  form 


FIG.  51.  —  Refraction  of  light  by  a  glass  prism.  A  beam  of  light  passing  through  an 
aperture  at  a  would,  if  uninterrupted,  fall  upon  the  screen  at  d.  If,  however,  it  is  inter- 
rupted by  the  prism  b,  it  is  bent  out  of  course,  i.e.  refracted,  and  falls  upon  the  screen  at 
a  lower  point.  At  the  same  time  the  different  rays  composing  the  beam  are  refracted 
differently,  thus  producing  a  spectrum. 


of  electric  discharge  to  flow  from  points  of  the  solid  to  be  tested,  or  over 
the  surface  of  the  liquid. 

Gases  may  be  made  luminous  by  passing  an  electric  discharge  through 
glass  tubes  containing  minute  quantities  of  them. 

4.  Highly  heated  solids  and  liquids  in  general  give  out  white  light  that 
is  not  characteristic,  but  is  the  same  for  all  of  them. 


CERTAIN  FORMS  OF  ENERGY.  89 

5.  Highly  heated  vapors  and  gases  in  general   give  out  light  having 
colors  that  are  peculiar,  and  characteristic  of  the  atoms  present. 


FIG.  52.  —  Adjustable  slit  for  use  upon  a  spectroscope. 

The  denser  the  vapor,  the  greater  the  light;   hence  metallic  vapors  give 
out  strongly  luminous  rays. 


FIG.  53.  —  Spectroscope  (of  one  prism)  for  the  examination  of  luminous  flames. 


6.  When  light  falls  upon  portions  of  matter,  it  is  capable  of  at  least 
three  different  dispositions :  — 

Certain  substances  allow  nearly  all  the  rays  of  a  complex  beam  of  light 
to  pass  through  them. 


9o 


CERTAIN  FORMS  OF  ENERGV. 


Other  bodies  absorb  certain  rays  and  allow  certain  others  to  pass 
through  them;  the  fact  of  such  absorption  is  shown  sometimes  by  the 
color  of  the  body  itself  or  sometimes  by  the  spectroscope. 

A  third  class  contains  bodies  that  are  opaque;  that  is,  they  forbid  any 
rays  of  light  to  pass  through  them,  but  they  sometimes  give  back  reflected 
light  of  a  peculiar  color. 


FIG.  54.  —  Apparatus  for  examining  the  spectrum  of  a  liquid.  Upon  the  passage  of  an 
electric  current  through  the  apparatus,  the  liquid  is  highly  heated,  and  affords  a  luminous 
mass  between  the  points  AB.  Thereupon  this  mass  may  be  examined  by  the  spectroscope. 


The  spectroscope  is  capable  of  showing  whether  light  as  originally 
emitted  has  been  modified  by  the  body  upon  which  it  has  fallen. 

As  a  result  of  the  study  of  spectrum  analysis  it  is  believed  that  the  light 
which  characterizes  each  element  is  due  to  the  atomic  motion  peculiar  to 
that  element.  In  fact,  certain  elements  form  colored  compounds  such  as 
give  similar  spectral  lines  whether  heated  or  cold,  and  so  appear  always  to 
maintain  the  same  rate  of  atomic  motion  (didymium). 


CERTAIN  FORMS  OF  ENERGY.  9 1 

That  light  is  a  form  of  motion  is  further  sustained  by  the  fact  that  spec- 
trum lines  appear  displaced,  owing  to  the  rapid  advance  toward  or  retire- 


FIG.  55. — Geissler  tube.  It  is  so  constructed  that  a  portion  of  gas  contained  in  it 
mny  be  highly  heated  by  an  electric  current.  The  narrow  portion  of  the  tube  may  be 
placed  before  the  slit  of  the  spectroscope  for  examination. 

ment  from  the  earth  of  certain  comets  and  other  celestial  bodies.  This 
displacement  in  the  spectrum  is  analogous  to  the  change  of  pitch  of  rapidly 
moving  sonorous  bodies,  as  the  whistles  on  moving  locomotives. 


FIG.  56.  —  Direct  vision  spectroscope,  employed  to  examine  a  colored  liquid.     The 
kind  of  dye-stuff  in  a  liquid  is  often  determined  by  this  method  of  examination. 

(/)   Work.  —  Heat  may  accomplish  external  or  internal  work.     The 
former  product  is  evident  to  us  in  various  visible  forms  of  mass-motion. 


92  CERTAIN  FORMS  OF  ENERGY. 

Internal  work  is  done  when  some  natural  molecular  force  is  over- 
come. Thus  this  is  accomplished  when  the  cohesive  power  of  a  solid  is 
so  overpowered  that  liquid  is  produced.  Ice  when  melted  occupies  a 
diminished  bulk,  so  that  in  this  case  no  external  work  results. 


FIG.  57.  —  Spectroscope  attached  to  a  telescope  for  the  purpose  of  examining  the  pro- 
tuberances on  the  outside  of  the  disk  of  the  sun.  (The  portrait  represents  J.  Norman 
Lockyer,  the  celebrated  English  astronomer.) 


The  internal  work  of  heat  is  explained  by  supposing  that  in  this  case 
there  is  produced  some  internal  motion,  like  rotation  of  molecules,  for  exam- 
ple, rather  than  one  like  a  translation  of  them. 


CERTAIN    FORMS    OF    ENERGY.  93 

ELECTRICITY. 

The  phenomena  of  electricity  already  known  are  so 
numerous,  varied,  and  subtile  as  to  defy  complete  ex- 
planation. Of  the  various  theories  of  its  nature  that 
have  been  suggested,  none  seem  on  the  whole  so  satis- 
factory as  that  it  is  some  form  of  energy ;  in  other  words, 
a  form  of  motion.  The  exact  character  of  this  motion 
cannot  at  present  be  stated.  It  is  probable  that  in  some 
cases  it  is  a  motion  of  translation  of  molecules  ;  oftener, 
perhaps,  it  is  a  motion  of  mere  rotation  or  similar  polar- 
ization of  molecules  without  change  of  position.  Many 
of  the  phenomena  seem  to  demand  the  intervention  of 
some  medium  like  the  ether,  —  whether  the  same  as 
that  supposed  for  the  explanation  of  radiant  heat  or 
only  a  somewhat  similar  one,  it  is  impossible  to  declare. 
However  objectionable  the  theory  of  an  ether  —  or  of 
one  or  more  fluids  —  is,  yet  in  the  present  state  of 
knowledge  something  of  the  sort  seems  indispensable 
for  purposes  of  explanation. 

Note  the  tendency  to  use  such  expressions  as  "the 
electric  current."  That  there  is  any  current  of  matter  is 
not  probable ;  but  there  certainly  is  a  progressive  trans- 
fer of  electric  energy.  This  transfer  is  marked  by  a 
velocity  nearly  double  that  of  light. 

Its  Sources.  —  It  is  a  very  suggestive  fact  that  what- 
ever known  source  of  electricity  is  invoked,  there  is 
always  an  evident  consumption  of  force.  This  becomes 
apparent  from  a  mere  enumeration  of  some  of  its  direct 
sources. 

Such  sources  are :  the  energy  of  friction;  the  energy  of  mere  mechani 
cal  separation  of  certain  bodies  in  contact;  mere  application  of  heat; 


90  CERTAIN  FORMS  OF  ENERGY. 

mere  change  of  temperature;   mechanical  separation  of  magnetically  at- 
tracted bodies;   chemical  combination  and  decomposition. 

The  modern  system  of  producing  electricity  for  purposes  of  illumina- 
tion illustrates,  in  an  interesting  manner,  several  transformations  of  force. 


FIG.  60.  —  Electric  light  produced  by  a  Bunsen  battery  and  employed  in  a  magic 
lantern  for  the  purpose  of  projecting  the  image  of  a  microscopic  object  upon  the  screen. 


This  system  generally  includes  at  least  four  contrivances:  first,  the  fire-box; 
second,  the  boiler  and  engine;  third,  the  dynamo;  fourth,  the  lamp.  In 
the  fire-box,  energy  of  the  chemical  affinity  of  burning  coal  is  transformed 
into  the  energy  of  heat;  in  the  boiler  and  engine  the  energy  of  heat  is 


CERTAIN  FORMS  OF  ENERGY. 


97 


transformed  into  the  energy  of  mass-motion;  in  the  dynamo  the  energy  of 
mass-motion  is  transformed  into  the  energy  of  electricity;  in  the  lamp,  the 
energy  of  electricity  is  transformed  into  the  energy  of  light  and  heat. 

Its  Effects.  —  That  the  effects  of  electricity  are  strik- 
ingly suggestive  of  some  form  of  motion  is  evinced  by  a 
brief  reference  to  a  few  of  them. 


FIG.  61.  —  Method  of  precipitating  silver  or  gold  from  a  solution  of  the  metal  upon 
an  ornamental  object.  In  this  case  the  electric  current  from  a  Bunsen  battery  is  em- 
ployed. The  current  from  a  dynamo  will  also  serve. 


(a)  One  of  the  most  early  and  easily  observed  of  the  effects  is  mechan- 
ical motion;  thus  electrified  bodies  readily  exhibit  direct  attraction  and 
repulsion  of  mass.  Mechanical  apparatus  can  also  be  kept  in  motion  by  a 
proper  application  of  electricity. 

(£)  Electricity  gives  rise,  by  direct  transformation,  to  other  admitted 
forms  of  force,  such  as  heat,  light,  and  magnetic  polarity. 

(c)  Electricity  produces  motion  in  chemical  molecules.  In  the  pro- 
cesses of  electro-plating  and  other  forms  of  electrolysis  it  effects  a  drawing 


98  CERTAIN  FORMS  OF  ENERGY. 

apart  of  portions  of  matter  previously  firmly  bound  together  by  chemical 
affinity. 

(</)  The  very  peculiar  phenomena  of  conduction,  insulation,  and  electric 
induction,  seern  to  favor  the  view  here  presented;  for  it  is  hardly  possible 
to  conceive  of  such  results  springing  from  a  transfer  of  matter,  even  of  a 
most  tenuous  kind. 


CHAPTER    IX. 
THE   ATTRACTIONS  OF  MASSES. 

GRAVITATION. 

GRAVITATION  is  a  force  by  which  considerable  por- 
tions of  matter  are  mutually  attracted,  whatever  their 
size,  nature,  distance  apart,  or  the  intervening  medium. 

Law.  —  The  gravitating  attraction  between  material 
objects  is  directly  proportional  to  their  masses,  and  in- 
versely proportional  to  the  squares  of  the  distances  between 
their  centres  of  gravity. 

In  accordance  with  this  law,  then,  if  two  bodies,  each 
containing  a  unit  of  mass,  gravitate  toward  each  other 
with  a  certain  force,  then  when  one  of  those  bodies  has 
its  mass  doubled  or  trebled,  the  gravitating  force  is  mul- 
tiplied by  two  or  three  ;  and  if  at  once  the  mass  of  one 
body  is  doubled,  and  the  mass  of  the  other  is  trebled, 
then  the  gravitating  force  is  doubled  by  reason  of  the 
one  increase,  and  trebled  by  reason  of  the  other ;  that 
is,  it  is  increased  in  the  proportion  of  2  X  3  =  6  times. 

Again,  if  the  masses  remain  unchanged,  and  the  dis- 
tance between  the  centres  of  gravity  of  the  bodies  is 
increased  to  two  or  to  three  units  of  distance,  then  the 
gravitating  force  is  reduced  respectively  to 


H-ortoH- 


99 


IOO  THE    ATTRACTIONS    OF    MASSES. 

If  the  distance  stated  is  reduced  to  one- half  or  one- 
third,  then  the  gravitating  force  is  increased  to  four 
times  or  nine  times,  respectively. 

Gravitation  is  the  great  agent  of  stability  in  nature. 
It  regulates  the  motions  of  the  heavenly  bodies.  By  its 
influence  objects  on  the  surface  of  the  earth  retain  their 
positions  instead  of  being  cast  forth  into  space. 

Its  value  is  best  estimated  by  considering  the  results 
which  would  follow  its  suspension,  supposing  the  latter 
to  be  possible  —  though  in  fact  it  is  not. 


CHAPTER    X. 

THE  ATTRACTIONS   OF  MOLECULES. 

I.  — COHESION. 

COHESION  is  an  attractive  force  acting  at  insensible 
distances  between  molecules  of  the  same  kind.     Besides 


FIG.  62.  — Wire-drawing  machine.  The  coarser  wire  on  the  reel  A  passes  through  a 
small  opening  in  the  steel  plate  f.  It  is  wound  as  a  finer  wire  upon  the  drum  B.  The 
operation  illustrates  a  resistance  of  solids  to  change  of  form.  (The  metal  remains  solid 
during  the  operation.) 

this  cohesive  force,  molecules  of  the  same  kind  are  influ- 
enced by  an  expansive  tendency  (undoubtedly  due  to 
heat). 

In  solids  the  cohesive  force  manifests  itself  in  the 
resistance  they  offer  to  any  derangement  of  the  shape 
of  the  solid ;  that  is,  of  the  arrangement  of  molecules 


'162''      '  THE' ATTRACTIONS  OF  MOLECULES. 

within  the  mass.  Thus  solids  offer  resistance  to  in- 
crease or  decrease  of  volume,  also  to  flexure  and  to 
rupture,  and,  indeed,  to  any  alteration  of  shape,  even 
though  not  involving  increase  or  decrease  of  bulk. 

In  gases  the  opposite  extreme  is  found.  They  seem 
to  possess  little  or  no  cohesive  force ;  the  expansive 
tendency  predominates.  A  few  cubic  inches  of  gas 
placed  in  a  vacuous  receiver,  of  any  shape,  and  of  any 
size  (not  exceeding  forty  or  fifty  miles  in  height),  would 
doubtless  soon  expand  so  as  to  fill  the  receiver.  Appar- 
ently the  remoteness  of  the  molecules  of  a  gas  from  one 
,  another  is  an  important  feature ;  they  are  thereby  re- 
moved more  from  the  influence  of  their  specific  cohesive 
forces,  and  thus  are  made  capable  of  greater  freedom  of 
motion. 

Liquids  exist  under  conditions  that  are  in  a  certain 
sense  intermediate  between  those  of  solids  and  of  gases. 

Liquids  manifest  cohesion  in  their  tendency  to  as- 
sume the  globular  form  (that  form  affording  the  most 
compact  arrangement  for  a  given  number  of  centrally 
attracted  objects). 

Liquids  display  the  existence  of  the  expansive  ten- 
dency in  their  easy  evaporation. 

Polarity.  — There  is  a  striking  difference  between 
solids  and  liquids  as  to  the  adjustments  of  the  molec- 
ular forces.  In  liquids  the  cohesive  forces  seem  to  be 
balanced  about  the  centres  of  the  molecules,  so  that 
these  molecules  are  free  to  move  about  each  other,  and 
to  occupy  equally  well  many  positions  with  respect  to 
each  other.  In  solids,  on  the  other  hand,  there  exists 
polarity ;  i.e.  the  cohesive  forces  seem  to  reside  out  of 


THE  ATTRACTIONS  OF  MOLECULES.        IO3 

the  centres  of  the  molecules,  and  in  certain  centres  of 
force  which  may  be  called  poles. 

This  polarity  not  only  compels  solids  to  offer  that 
resistance  to  derangement  of  shape  already  referred  to. 
It  also  produces  the  phenomena  of  crystallization. 

Crystallization.  —  In  general,  a  portion  of  matter  in 
the  act  of  changing  from  the  gaseous  or  the  liquid  to 
the  solid  form  manifests  a  tendency  toward  a  definite 
arrangement  of  molecules.  With  the  exception  of  a  few 
animal  and  vegetable  products  every  solid  affects  a  defi- 
nite polyhedral  form,  although  it  may  manifest  this  ten- 
dency only  under  favorable  circumstances.  A  body 
possessing  such  a  form  is  called  a  crystal. 

It  seems  proper  to  assume  that  the  crystalline  condi- 
tion is  the  normal  one  for  all  solids. 

It  is  true  that  certain  distinctively  animal  and  vege- 
table matters  —  the  so-called  organized  matters  —  as- 
sume the  cellular  form  rather  than  the  crystalline.  They 
are  not  as  thoroughly  exceptional,  however,  as  might  at 
first  appear.  (See  p.  105.) 

Cleavage.  —  Certain  well-crystallized  substances,  when 
broken,  are  found  to  possess  the  property  of  cleavage  in 
a  marked  degree  :  if  pulverized,  or  otherwise  subdivided, 
they  undergo  fracture  with  far  greater  ease  in  certain 
directions  than  in  others.  A  given  portion  of  Iceland 
spar,  for  example,  having  a  well-defined  rhombohedral 
form,  may  be  broken  up  into  smaller  rhombohedrons 
rather  than  into  masses  of  indefinite  shape.  So  a  mass 
of  galena,  if  broken,  forms  rectangular  or  cubical  masses. 
Even  when  these  substances  are  pulverized,  examination 


IO4  THE    ATTRACTIONS    OF    MOLECULES. 

by  the  microscope  shows,  in  each  case,  the  strongly 
marked  crystalline  tendencies ;  each  particle  of  powder 
shows  itself  to  be  a  little  crystal,  or  mass  of  crystals. 
The  same  principle  is  observed  in  other  crystalline  sub- 
stances, though  perhaps  in  less  marked  degree. 

In  general,  it  may  be  assumed  that  any  crystalline  substances  when 
broken  up  into  small  particles  will  show  crystalline  fracture,  and  in  the 
powder  will  show  a  multitude  of  little  crystals  bearing  a  strong  resemblance 
to  the  large  crystal  whence  it  came.  The  cutting  of  diamonds  and  other 
precious  stones  depends  upon  this  general  principle;  i.e.  that  layers  may 
be  cleaved  off  in  certain  directions  better  than  in  others.  It  is  very  plain, 
then,  that  a  crystal  possesses  not  merely  external  symmetry  —  the  laws  of 
its  structure  govern  most  thoroughly  its  internal  parts.  It  may  be  imag- 
ined that,  if  it  were  practicable  to  reduce  the  size  of  a  given  large  crystal 
by  removal  of  its  outer  layers,  one  by  one,  the  time  would  come  when,  the 
limit  of  the  single  molecule  being  reached,  this  molecule  itself —  if  capable 
of  remaining  solid  —  would  be  found  to  possess  something  equivalent  to  a 
crystalline  form.  It  would  be  either  a  miniature  of  the  large  crystal  or 
else  one  geometrically  related  to  it. 

Theoretically,  at  least,  it  may  be  considered  that 
a  single  molecule  first  solidifies,  and  then  other  mole- 
cules build  themselves  upon  it  as  upon  a  nucleus.  It 
may  be  assumed  that  they  pile  themselves  up  upon  these 
outer  faces  in  accordance  with  some  definite  geometrical 
law.  These  statements  strengthen  the  conviction  that 
crystalline  form  is  not  a  mere  external  and  superficial 
peculiarity  of  substances,  but  is  the  product  of  a  funda- 
mental and  essential  law  of  them.  Probably  it  is  closely 
connected  with  their  atomic  and  molecular  constitution. 
Mitscherlich's  law  (see  p.  234)  represents  an  attempt  to 
deal  with  this  relationship.  It  may  be  expected  that  in 
the  future  yet  other  and  more  definite  connection  will 
be  shown  between  the  crystalline  form  and  chemical 
constitution. 


THE  ATTRACTIONS  OF  MOLECULES.        IO5 

Apparent  Exception.  —  It  is  commonly  accepted  as  a  principle  that 
in  a  certain  sense  organized  bodies  (see  p.  154)  do  not  conform  to  these 
statements.  Perhaps  in  another  sense  they  will  ultimately  be  found  in 
harmony  with  it.  Thus  certain  organized  bodies,  like  muscle,  are  made 
up,  not  of  one  substance,  but  of  several  different  substances,  arranged  in 
the  cellular  form.  If  these  substances  could  be  separated  one  from  an- 
other, and  then  each  reduced  to  the  solid  form,  perhaps  they  would  then 
appear  as  a  set  of  crystalline  compounds.  This  latter  statement  is  made 
with  the  general  admission,  already  presented  in  another  place,  that  differ- 
ent substances  well  recognized  as  crystallizable  assume  the  crystalline  form 
with  different  degrees  of  ease.  In  other  words,  in  order  to  crystallize,  sub- 
stances demand  conformity  to  many  conditions. 

Crystalline  Systems.  —  The  various  crystalline  forms 
recognized  have  been  classified  in  six  so-called  systems  ; 
and,  moreover,  a  substance  crystallizing  in  a  given  sys- 
tem is  capable  of  certain  variations  within  that  system. 
In  this  way  there  exist  within  a  given  system  not  only 
the  simple  forms,  but  also  what  are  called  compound 
forms  and  hemihedral forms. 

Certain  substances  crystallize  in  shapes  that  are 
readily  recognized  and  referred  to  their  proper  sys- 
tems. Others  assume  such  complicated  forms,  or  else 
such  imperfectly  developed  shapes,  or  else  crystallize  in 
such  minute  portions,  that  their  recognition  is  difficult. 
Sometimes  a  special  apparatus  called  a  goniometer  is 
employed  to  determine  the  particular  crystalline  form. 

The  several  systems  are  presented  in  brief  but  com- 
pact form  in  the  following  tables  and  diagrams  :  — 


io6 


THE  ATTRACTIONS  OF  MOLECULES. 


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IO8  THE    ATTRACTIONS    OF    MOLECULES. 

The  Process  of  Crystallization.  -  -  The  process  of 
crystallization  is  a  variety  of  the  process  of  solidifica- 
tion ;  but  it  is  one  that  is  dependent  upon  a  highly 
specialized  arrangement  of  the  particles  of  the  solid. 
Hence  it  must  be  supposed  that  solidifying  molecules 
take  up  certain  determinate  motions  before  they  have 
placed  themselves  in  those  symmetrical  and  orderly 


FIG.  69.  —  Crystals  formed  in  a  mass  of  metallic  bismuth  by  slow  cooling 
of  the  melted  metal. 

ranks  required  by  the  crystalline  condition.  This 
special  kind  of  motion  is  not  consistent,  however,  with 
the  two  fluid  states  of  matter,  nor  yet  with  the  rigid 
one.  It  seems  to  best  take  place  in  that  transition 
period  which  extends  between  them.  It  might  be 
expected,  then,  that  crystallization  would  be  facilitated 
by  increasing  as  far  as  possible  the  extent  of  this 
transition  state.  This  is  found  to  be,  in  fact,  the  case. 
Whenever  the  progress  of  solidification  is  prolonged, 
the  tendencies  toward  crystallization  are  favored. 


THE  ATTRACTIONS  OF  MOLECULES. 


Crystals  are   usually  produced    by 
the  following  means  :  — 

(a)    The    Slow  Cooling  of  Vapors. — 

The  formation  of  crystals  of  snow 
from  water-vapor  in  the  atmosphere 
is  an  example  of  this  method. 

The  formation  of  crystals  of  iodine 
is  another  example. 


FIG.  70.  —  Section  of  a 
crucible  in  which  melted 
sulphur  has  been  allowed 
to  crystallize  by  slow 
cooling. 


(b)  The  Slow  Cooling 
of  Liquids  produced  by 
Fusion.  —  The  crystalli- 
zation of  melted  sulphur, 


FIG.  71.  —  Diagram  showing  crystalline  form 
assumed  by  sulphur  during  slow  cooling  from 
the  melted  form. 

of  melted  bismuth,  and  of 
melted  zinc  are  examples  of 
this  method. 

(c)   The  Slow  Cooling  of  Liq- 
uids produced   by  Solution.  — 

This  is  the  method  by  which 
most  crystals  produced  in  the 
arts  are  formed.  A  very  fa- 
miliar example  is  rock  candy, 

which  is  crystallized  by  cooling  the  liquid  produced  by 
dissolving  cane  sugar  in  hot  water. 


FIG.  72. —  A  diagram  showing  crys- 
talline form  in  which  sulphur  is  found 
in  nature. 


IIO        THE  ATTRACTIONS  OF  MOLECULES. 

Multitudes  of  crystalline  salts  are  manufactured  by 
chemists  by  this  method. 


FIG.  73.  —  Method  of  producing  crystals  of  rock  candy  by  slow  cooling  of  the  solution 
of  cane  sugar  in  water.     The  crystals  collect  upon  threads  stretched  through  the  liquid. 


(d)    Slow  Evaporation  of  Liquid  Solutions.  —  The  ma- 
jority of  solid  chemical  salts  known  may  be  dissolved  in 


FIG.  74.  —  Crystals  of  potassic  nitrate  (saltpetre)  formed  by  the  slow  cooling 
of  a  solution  of  the  salt  in  boiling  water. 

water  and  thus  changed  temporarily  to  the  liquid  form. 
If  such  a  liquid  is  evaporated,  the  water  may  be  expelled 


11 


II 


in 


112  THE   ATTRACTIONS    OF    MOLECULES. 

and  the  solid  caused  to  reappear.  If  the  expulsion  of 
the  water  is  conducted  very  slowly,  the  solid  reappears 
so  slowly  that  its  molecules  have  time  to  arrange  them- 
selves in  the  form  of  crystals. 

An  example  of  this  method  is  found  in  the  manufac- 
ture of  common  salt,  which  is  generally  crystallized  from 
its  solution  in  water,  by  means  of  slow  evaporation. 


FIG.  76.  —  Rock  crystals  found  in  nature.     The  substance  is  silicic  oxide  (SiO2). 
forms  in  hexagonal  prisms  surmounted  by  hexagonal  pyramids. 


Theoretically,  a  crystal  once  formed  in  a  solution  and 
continuing  to  increase  in  size  (by  every  face  of  every 
set  of  faces  receiving  a  deposit  of  the  same  thickness) 
would  produce  an  ideally  perfect  crystal.  But  as  a  fact, 
distortion  almost  always  results.  By  the  change  in 
specific  gravity  in  the  liquid,  from  loss  of  the  solid 
substance,  currents  are  generated,  and  different  parts  of 


THE  ATTRACTIONS  OF  MOLECULES.        113 

the  crystal  are  subjected  to  different  conditions.  Again, 
by  the  crystal  becoming  attached  to  other  crystals,  or  to 
the  side  or  bottom  of  the  vessel,  the  several  faces 
receive  unequal  deposits  ;  yet  every  face  always  remains 
parallel  to  its  original  position  and  the  interfacial  angles 
are  constant. 


CHAPTER   XL 

THE  ATTRACTIONS  OF  MOLECULES  (continued}. 
II.  — ADHESION. 

ADHESION  is  a  form  of  attractive  force,  exerted  at  in- 
sensible distances,  between  molecules  of  different  kinds. 

(A)  ADHESION  BETWEEN  SOLIDS  AND  SOLIDS. 

Of  this  kind  of  adhesion  there  are  many  examples. 
The  adhesion  of  a  piece  of  wood  to  another  of  different 
kind  by  means  of  a  layer  of  glue  involves  two  illustrations  ; 
for  the  solidified  glue  adheres  to  each  kind  of  wood. 

The  rock  known  as  granite  is  composed  of  three 
different  and  separate  materials,  —  quartz,  felspar,  and 
mica,  —  easily  recognized  by  inspection  ;  they  are  held 
together  by  a  form  of  adhesion. 

There  are  certain  cases  in  which  two  solids  when  brought  in  contact 
liquefy,  or  set  up  some  very  evident  chemical  change.  Thus  when  solid 
ice  and  solid  salt  are  placed  together,  each  exerts  on  the  other  a  very 
remarkable  attractive  force  by  reason  of  which  the  ice  melts  and  the  salt 
dissolves  in  the  water  formed. 

The  phenomena  of  this  operation,  and  others  similar  to  it,  are  discussed 
more  properly  under  Dissolving  of  Solids  in  Liquids  (p.  118)  and  Chemi- 
cal Action  (pp.  169  and  171). 

(B)  ADHESION  BETWEEN  SOLIDS  AND  LIQUIDS. 

Of  this  kind  of  adhesion  there  are  many  forms  worthy 
of  consideration  here. 
114 


THE  ATTRACTIONS  OF  MOLECULES. 


FIRST  FORM.  —  Moistening. 

This  form  is  exemplified  by  many  solids  and  liquids. 
Thus,  a  glass  rod  dipped  in  water  and  then  withdrawn  is 
found  to  retain  some  water  upon  its  surface. 


FIG.  77.  —  Disposition  of  apparatus  for  showing  the  adhesion  of  a  solid  to  the  surface 
of  a  liquid.  The  upper  part  of  the  figure  represents  one  pan  of  the  balance.  In  the  other 
pan  of  the  balance  (not  shown  in  the  cut)  weights  may  be  added  until  the  plate  (shown  at 
the  bottom  of  the  cut)  is  pulled  away  from  contact  with  the  liquid. 

SECOND  FORM.  —  Capillary  Attraction. 

When  a  tube,  open  at  both  ends,  is  dipped  into  liquid 
contained  in  a  considerably  larger  vessel,  tnere  may  be 
three  cases  (under  this  title). 


n6 


THE  ATTRACTIONS  OF  MOLECULES. 


The  case  oftenest  observed  is  that  in  which  the  liquid 
in  the  tube  rises  to  some  distance  above  the  general 
level  of  the  liquid  in  the  vessel.  A  tube  of  glass  dipped 
in  water  displays  this  phenomenon. 

In  some  cases  the  liquid  in  the  tube  is  depressed 
below  the  level  of  that  in  the  vessel.  A  glass  tube 
dipped  in  mercury  affords  an  illustration  of  this  case. 

It  might  be  expected  that  the  case  of  a  liquid  main- 
taining the  same  level  within  and  without  the  tube 


FIG.  78.  —  Capillary  attraction,  showing  rise 
of  liquid  in  narrow  tubes. 


FIG.  79.  —  Capillary  depression,  shown  by 
fall  of  mercury  in  a  narrow  tube  of  glass. 


would  be  a  rare  one ;  for  this  can  only  exist  when  there 
prevails  a  certain  exact  balance  between  the  amount  of 
cohesion  of  the  liquid  itself,  and  double  the  amount  of 
the  adhesive  force  of  the  liquid  to  the  material  of  the 
tube.  Of  course  exact  equality  is  everywhere  an  excep- 
tional condition  of  things. 

THIRD  FORM.  —  Spheroidal  State. 

When  a  small  portion  of  liquid  is  placed  upon  the 
surface  of  a  supporting  material  that  is  relatively  highly 
heated,  the'  liquid  draws  itself  up  into  a  globule  and 


THE  ATTRACTIONS  OF  MOLECULES. 


117 


moves  about  in  what  is  called  the  spheroidal  state. 
The  heated  surface  causes  the  liquid  to  evaporate 
chiefly  on  its  under  side ;  the  abundant  vapors  thus 


FIG.  80.  —  Experiment  to  illustrate  the  spheroidal  state  of  water.  The  lamp  heats  the 
plate,  whereupon,  if  drops  of  water  are  placed  upon  it,  they  remain  as  little  spheres  and 
do  not  adhere  to  the  plate. 


produced  afford  a  cushion  upon  which  the  liquid  is 
supported.  Of  course  the  liquid  does  not  rest  in  actual 
contact  with  the  heated  material. 


Il8  THE   ATTRACTIONS    OF    MOLECULES. 

A  noticeable  anomaly  exists  in  the  case  described.  Even  upon  a  solid 
surface  of  a  very  high  temperature  the  liquid  does  not  evaporate  as  rapidly 
as  in  a  vessel  sustained  at  a  much  lower  temperature.  But  at  these  lower 
temperatures  the  liquid  rests  in  contact  with  the  solid,  and  the  entire  mass 
of  liquid  receives  heat  by  the  processes  of  conduction  and  convection. 
At  very  high  temperatures,  however,  the  liquid  is  not  in  contact;  the 
globule  then  receives  heat  just  as  other  objects  do  at  a  great  distance  from 
a  source  of  heat;  that  is,  by  the  process  called  radiation.  By  this  means, 
the  lower  surface  of  the  globule  is  the  portion  chiefly  influenced;  here 
vapors  are  given  off  in  abundance  —  sufficient  to  afford  the  supporting 
cushion,  but  not  sufficient  to  rapidly  diminish  the  mass  of  the  globule. 

FOURTH  FORM.  —  Sohition  of  a  Solid  in  a  Liquid. 

The  dissolving  of  one  or  more  liquids  may  involve  the 
interaction  of  a  great  many  forces,  so  that  solution  in 
its  more  complex  varieties  is  worthy  of  careful  study 
and  extended  discussion. 

The  general  opinion  now  prevails  that  substances  in  dilute  solutions 
exist  in  a  condition  somewhat  analogous  to  substances  in  the  gaseous  state 
under  moderate  pressure  and  moderately  high  temperature.  'This  view  is 
based  upon  the  studies  of  J.  H.  van  't  Hoff  and  F.  Raoult. 

Even  in  the  simplest  forms  the  process  of  solution 
depends  on  a  variety  of  conditions. 

The  amount  of  a  given  solid  capable  of  dissolving  in 
a  liquid,  in  a  given  experiment,  depends  upon  at  least 
three  factors  :  — 

The  nature  of  the  solid  and  the  liquid ; 

The  quantity  of  the  solid  and  the  liquid ; 

The  temperature  under  which  the  experiment  is 
conducted. 

The  rapidity  with  which  in  a  given  experiment  any 
certain  (possible)  amount  of  a  solid  is  dissolved  in  a 
given  amount  of  liquid,  depends  upon  the  rapidity  with 
which  the  favorable  conditions  are  provided.  Thus 


THE  ATTRACTIONS  OF  MOLECULES.        I IQ 

extreme  fineness  of  comminution,  agitation  of  the  mix- 
ture of  solid  and  liquid,  rapid  addition  of  heat, — all 
favor  rapid  dissolving. 

I .  Nature  of  the  Liquid  and  the  Solid.  —  (a)  Water 
is  specifically  gifted  with  solvent  power  of  a  remarkably 
wide  range.  Moreover,  it  is  a  very  abundant  and  widely 
diffused  substance.  In  many  ways  it  seems  entitled  to 
be  considered  the  chief  liquid.  Now  water  dissolves  in 
large  quantities  a  very  large  number  of  chemical  salts. 
As  examples  may  be  mentioned,  potassic  sulphate, 
K2SO4 ;  sodic  sulphate,  Na2SO4  •  10  H2O  ;  potassic  ni- 
trate, KNO3;  zinc  sulphate,  ZnSO4-  7  H2O  ;  sodic  chlor- 
ide, NaCl.  In  fact,  it  dissolves  in  greater  or  smaller 
quantity  the  majority  of  salts  known.  Water  also  dis- 
solves a  great  number  of  neutral  bodies,  of  which  cane 
sugar  (CtfHffiOu)  may  be  used  as  an  example. 

(b)  Carbon   disulphide    dissolves    sulphur   and  some 
other  substances  not  soluble  in  water. 

(c)  Liquid    oils  dissolve  many  solid   fats ;    thus  the 
more  liquid  paraffins  dissolve  the  more  solid  ones  like 
white  paraffin  wax. 

(d}  Liquid  mercury  dissolves  most  of  the  metals,  as 
potassium,  sodium,  gold,  silver,  zinc  (but  not  iron). 
Mercurial  solutions  and  their  more  solid  forms  are 
called  amalgams. 

(e)  Melted  zinc  dissolves  many  metals,  as  solid  cop- 
per, gold,  platinum,  and  others.  Products  of  this 
general  character  both  before  and  after  solidification 
are  called  alloys. 

(/)  Diluted  sulphuric  acid  dissolves  metallic  zinc  and 
other  metals. 


I2O 


THE    ATTRACTIONS    OF    MOLECULES. 


Remarks  on  this  List.  —  This  list  is  merely  one  of  examples.  Other 
examples  might  have  been  given  under  each  head,  and  perhaps  with  equal 
propriety. 

In  examples  l>,  c,  d,  e,  it  plainly  appears  that  solvent  power  is  generally 
the  greater,  the  greater  the  similarity  of  the  solid  and  the  solvent  liqtiid. 

It  will  be  noted  that,  as  a  matter  of  course,  solvent  action  is  oftenest 
observed  in  cases  of  liquids  which  (like  water,  for  example)  remain  in  the 
liquid  form  at  the  temperatures  ordinarily  prevailing;  it  must  not  be  for- 
gotten that  if  the  globe  had  a  slightly  lower  climatic  temperature,  many  of 
these  would  be  best  known  as  solids;  they  would  be  thus  reduced  to  the 
category  of  those  (like  metallic  zinc,  for  example)  which  now  have  to  be 
artificially  raised  a  little  in  temperature  before  they  display  their  solvent 
powers.  Of  course,  at  greatly  reduced  temperatures,  all  known  liquids 
solidify,  and  would  be  thrown  out  of  the  account,  just  as  at  very  greatly 
elevated  temperatures  all  known  solids  would  probably  liquefy,  and  so 
would  come  into  the  list  of  liquid  solvents. 

Example  /  needs  special  attention.  In  a  very  just  sense  it  belongs  to 
case  a.  The  solvent  action  is  properly  that  of  water  upon  a  chemical  salt 
—  zinc  sulphate  (ZnSO4  •  7  H2O).  For  it  happens  that  the  dilute  sulphuric 
acid  exerts  such  a  chemical  action  upon  the  metallic  zinc  as  changes  it 
into  the  chemical  salt  —  zinc  sulphate.  When  dilute  sulphuric  acid  acts 
upon  metallic  zinc,  the  operation  may  be  represented  as  follows :  — 


Zn        +          H2SO4 

f          7H20 

+            Aq 

One  atom  of            One  molecule  of 

Seven  molecules  of 

Indefinite  amount  of 

Zinc,              Sulphuric  acid, 

Water, 

Water. 

65                            98 

126 

parts  by  weight.         parts  by  weight. 

parts  by  weight. 

289 

ZnSO4    7  H2O               + 

H2 

+               Aq 

One  molecule  of 

One  molecule  of 

Indefinite  amount  of 

Crystallized  zinc  sulphate, 

Hydrogen, 

Water. 

287 

2 

parts  by  weight. 

parts  by  weight. 

289 

Then  the  zinc  sulphate  (ZnSO4  •  7  H2O)  dissolves  in  the  water  present 
in  the  original  dilute  sulphuric  acid. 

It  ought  to  be  noted  that  it  seems  highly  probable  that  in  all  cases  of 
solution  —  even  those  where  a  single  solid  dissolves  in  a  single  liquid  — 


THE  ATTRACTIONS  OF  MOLECULES.        121 

there  is  some  chemical  action.  In  cases  at  one  extreme  of  the  series  the 
action  may  be  very  marked.  This  is  so,  for  example,  when  sulphuric 
oxide  (SO3),  a  solid,  dissolves  in  water;  great  heat  is  here  evolved,  and 
there  is  produced  a  new  substance,  sulphuric  acid  (H2SO4),  which  also 
dissolves  in  water.  An  example  of  the  opposite  extreme  is  that  in  which 
sugar  dissolves  in  water.  It  would  be  difficult  in  such  a  case  to  demon- 
strate that  any  chemical  action  takes  place  unless  specially  devised  means 
were  employed  for  its  detection. 

2.  Influence  of  Change  of  Temperature.  —  The  com- 
prehension of  this,  as  well  as  of  other  branches  of  the 
subject,  may  be  facilitated  by  a  description  of  the  opera- 
tion of  solution  as  advancing  by  stages. 

When  a  solid  is  placed  in  a  liquid,  the  liquid  acts  first 
upon  the  outer  layers  of  the  solid. 

Adhesion  and  chemical  action  are  exerted  at  once. 
The  surface  molecules  of  the  solid  leave  the  others  and 
assume  the  liquid  form. 

The  liquefied  portions  from  the  original  solid  at  once 
diffuse  themselves  into  some  of  the  intermolecular  spaces 
of  the  bathing  liquid. 

True  solution  has  now  been  accomplished,  though  it 
may  as  yet  be  somewhat  limited  quantitatively. 

By  the  very  act  of  dissolving,  as  thus  far  described, 
there  have  been  created  certain  conditions  which  directly 
oppose  its  further  progress. 

The  first  of  these  opposing  conditions  is  a  reduced  temperature.  It  is  a 
recognized  law  that  in  all  changes  of  a  solid  to  a  liquid,  absorption  of  heat 
takes  place.  This  effect  is  recognized  in  the  cooling  of  whatever  happens 
to  be  the  surrounding  medium.  It  is  often  stated  that  in  liquefaction  sen- 
sible heat  becomes  latent  heat.  (While  the  expression  latent  heat  of  lique- 
faction is  a  well-established  one,  it  is  somewhat  inappropriate.  It  carries 
the  suggestion  that  a  certain  amount  of  heat  has  become  merely  concealed, 
whereas  heat  in  fact  ceases  to  be  —  as  heat  —  when  it  does  the  work  of  lique- 
fying a  solid.)  To  the  foregoing  should  be  added  another  important  state- 


122        THE  ATTRACTIONS  OF  MOLECULES. 

ment.  It  is  the  following :  The  amount  of  a  solid  that  a  liquid  can  hold 
in  solution  varies  with  the  temperature,  being  in  most  cases  the  greater  the 
higher  the  temperature.  It  follows  that  at  any  given  point  of  temperature 
the  liquid  may  hold  dissolved  a  certain  amount  of  the  solid  and  no  more. 
When  this  full  amount  is  in  fact  dissolved,  the  liquid  is  said  to  be  saturated. 
In  case  of  many  solids  and  liquids,  tables  have  been  constructed  showing 
by  the  graphical  method  the  quantities  producing  saturation  at  each  of  a 
series  of  temperatures. 

From  what  has  been  said  it  is  plain  that  if  it  is  desired  that  the  dissolv- 
ing operation  shall  go  on,  heat  must  be  added. 

The  second  of  the  opposing  conditions  is  the  local  saturation  of  the  sol- 
vent liquid.  By  reason  of  this  saturation  the  portions  of  liquid  lying  imme- 
diately about  the  solid  may  become  incapable  of  further  dissolving  action, 
while  more  remote  portions  may  not  as  yet  have  begun  to  act.  This  ces- 
sation is  sometimes  associated  with  the  fact  that  the  solid  rests  at  the 
bottom  of  the  liquid,  and  the  solution,  being  heavier  than  the  original 
liquid,  naturally  rests  upon  and  covers  the  solid.  There  are  three  ways  of 
overcoming  this  difficulty.  One  way  is  to  agitate  the  liquid  mechanically. 
A  better  way  is  to  suspend  the  solid  in  a  perforated  basket  hung  in  the 
upper  layers  of  the  liquid;  then  the  solution,  as  it  becomes  saturated,  sinks 
by  its  own  weight,  and  is  promptly  replaced  by  portions  of  fresh  liquid. 
A  third  method,  employed  to  advantage  in  combination  with  the  second, 
is  heating;  this  produces  convective  currents  in  the  liquid. 

3.  The  Quantity  of  the  Solid  and  Liquid.  —  This  point 
needs  no  discussion  in  addition  to  the  statements  relat- 
ting  to  saturation  already  introduced. 

Deliquescence.  —  This  is  a  form  of  solution.  It  is  ex- 
emplified by  certain  substances  that  have  such  a  strong 
attraction  for  water  that  they  even  absorb  that  moisture 
existing  as  vapor  in  the  atmosphere.  They  draw  this 
water  to  themselves  in  such  quantity  that  they  soon 
cease  to  be  solid ;  for  they  liquefy  by  dissolving  in  the 
water  absorbed. 

This  topic  is  naturally  associated  with  the  adhesion  of 
solids  and  gases.  (See  p.  130.) 


THE  ATTRACTIONS  OF  MOLECULES.        123 

Freezing  Mixtures.  —  In  some  cases  the  loss  of  heat 
associated  with  and  due  to  liquefaction  is  very  great. 
Thus,  when  ice  and  salt  are  mixed,  the  ice  melts  and 
the  salt  dissolves  in  the  water  so  formed.  Thus  both 
liquefy.  The  amount  of  heat  absorbed  from  surround- 
ing objects  is  very  great,  and  the  cold  so  produced  is 
utilized  in  many  operations  in  the  arts. 

The  mixture  remains  liquid  at  temperatures  much 
below  that  at  which  both  constituents  when  separate 
would  be  solid.  This  clearly  shows  that  the  liquefac- 
tion is  not  due  to  heat  alone,  but  involves  also  some 
specific  influence  of  adhesion  or  chemical  union  or  both 
together.  It  is  of  similar  nature  to  the  phenomena 
already  mentioned  under  the  title  eutexia.  (See  p.  49.) 

This  topic  has  certain  relations  to  the  adhesion  of 
solids  to  solids,  but  is  more  closely  affiliated  with  the 
solution  of  solids  in  liquids. 


CHAPTER    XII. 
THE  ATTRACTIONS  OF  MOLECULES  (continued). 

II.  —  ADHESION  (continued-}. 

THE  study  of  the  conditions  under  which  solids  dis- 
solve in  liquids  naturally  leads  to  a  consideration  of 
those  under  which  solids  may  be  separated  again  from 
liquids  holding  them  in  solution.  But  it  is  not  intended 
here  to  extend  the  discussion  to  the  formation  of  pre- 
cipitates by  sudden  chemical  reactions. 

THE  SEPARATION  OF  A  SOLID  FROM  A  LIQUID. 

The  comprehension  of  this  subject  may  be  facilitated 
by  a  few  typical  examples.  These  will  develop  the  fol- 
lowing simple  but  important  principle  :  As  dissolving  is 
favored  by  increase  of  quantity  of  solvent  and  by  addition 
of  heat,  so  separation  of  a  solid  from  its  solvent  is  favored 
by  decrease  of  quantity  of  liquid  and  by  decrease  of  heat. 
The  withdrawal  of  heat  is  almost  always  practicable  ;  the 
decrease  of  quantity  of  solvent  is  practicable  in  cases  of 
certain  liquids,  like  water,  carbon  disulphide,  alcohol, 
ether,  and  others  that  easily  evaporate. 

FIRST  EXAMPLE.  —  Cane  Sugar. 

If  a  saturated  aqueous  solution  of  cane  sugar  has  some  of  its  water 
removed  by  evaporation,  a  portion  of  sugar  corresponding  to  the  amount  of 
124 


THE  ATTRACTIONS  OF  MOLECULES. 


125 


water  so  removed  immediately  settles  out  in  the  solid  form.     Incidentally 
this  sugar  forms  crystals. 


FIG.  81.  —  The  vacuum-pan  for  producing  rapid  evaporation  of  water  from  sugar  solu- 
tions. The  vapor  as  fast  as  it  is  formed,  and  also  the  air  in  the  apparatus,  is  rapidly 
withdrawn  by  a  powerful  pump.  Whereupon  further  evaporation  takes  place,  and  the 
sugar  syrup  is  brought  to  a  condition  such  that  it  will  readily  crystallize. 

Again,  if  a  saturated  aqueous  solution  of  cane  sugar  is  reduced  in  tem- 
perature, a  portion  of  sugar  corresponding  to  the  amount  of  heat  with- 


126 


THE  ATTRACTIONS  OF  MOLECULES. 


FIG.  82.  —  Crystals  produced  in  a 
vessel  by  the  slow  evaporation  of  a 
liquid  produced  by  solution. 


drawn  immediately  settles  out  in  the  solid  form.     In  this  case,  also,  the 
solidifying  sugar  incidentally  crystallizes. 

Both  these  means  are,  in  fact,  employed  in  the  arts  for  the  manufacture 
of  sugar  on  the  large  scale. 

SECOND  EXAMPLE. — Alum. 

Potassic  sulphate  (K2SO4)  and  aluminic  sulphate  (A12[SO4]3)  may  be 
dissolved  together  in  water.     When  the  clear  solution  so  formed  is  either 

evaporated  or  cooled,  crystals  of  a  new 
double  salt  separate.  This  salt  is  called 
alum.  It  is  potassio-aluminic  sulphate, 
and  the  crystals  are  found  to  have  the 
composition  represented  by  the  formula 

K2S04,  A12(S04)3.24H20. 

Under  these  circumstances,  then,  the 
salts  have  the  power  of  drawing  to 

themselves,  by  reason  of  chemical  affinity  for  it,  a  definite  amount  of  water, 
called  in  this  case,  and  similar  ones,  water  of  crystallization.  Many  other 
salts  have  the  power  of  combining  chemically 
with  water  in  this  way.  In  fact,  it  is  believed 
to  be  probable  that  all  substances  that  dissolve 
in  water  combine  with  it  chemically,  though  the 
demonstration  of  the  fact  of  combination  some- 
times involves  difficulties. 

THIRD  EXAMPLE.  —  Sodic  Sulphate. 

Sodic  sulphate  presents  a  peculiar  and  inter- 
esting form  of  the  same  tendency  manifested  by 
alum;  i.e.  to  combine  with  water  under  proper 
conditions. 

Thus  there  exist  three  salts  —  the  same  in 
composition  except  as  respects  water  —  differing 
merely  according  to  the  circumstances  under 
which  they  solidify. 

These  salts  are :  — 


FIG.  83.  —  Burnt  alum. 
When  alum  is  heated  in  a 
crucible,  it  puffs  up  on  ac- 
count of  the  escape  of  water 
of  crystallization  in  the  form 
of  steam. 


Anhydrous  sodic  sulphate 
Heptahydrated  sodic  sulphate 
Dekahydrated  sodic  sulphate 


7  H2O. 
Na2SO4.  ioH2O. 


THE  ATTRACTIONS  OF  MOLECULES.        I2/ 

It  is  not  necessary  to  undertake  a  detailed  account  of  the  conditions 
under  which  these  several  compounds  are  produced.  It  is  sufficient  to 
state,  in  general,  that  the  formation  of  one  rather  than  another  is  a  matter 
of  temperature  mainly.  The  general  rule  is  that  in  solutions  of  lower 
temperatures  more  water  combines  with  the  salt;  in  solutions  of  higher 


FIG.  84.  — Dr.  Frederick  Guthrie,  lately  Professor  of  Physics  at  the  Royal  School  of 
Mines,  London.     Born  in  1833;  died  in  1886. 

temperatures  a  kind  of  dissociation  takes  place,  and  crystals  are  formed 
containing  less  water. 

FOURTH  EXAMPLE.  —  So  die  Chloride. 

Common  salt  affords  an  illustration  of  the  general  principle  just  illus- 
trated by  sodic  sulphate,  only  in  the  case  of  common  salt  the  principle  is 
extended  to  very  low  temperatures  indeed.  Crystals  of  common  salt 


128        THE  ATTRACTIONS  OF  MOLECULES. 

formed  at  ordinary  temperatures  are  anhydrous;  they  have  the  formula 
NaCl.  When  a  suitable  solution  is  cooled  considerably  below  the  freezing- 
point  of  water,  two  kinds  of  crystals  may  be  formed  —  one  variety  having 
the  formula  NaCl  •  2  H2O;  the  other  variety,  formed  at  still  lower  tempera- 
tures, having  the  formula  NaCl  •  io|H2O.  Crystals  formed  in  this  way 
below  the  freezing-point  of  water  are  called,  by  Guthrie,  cryohydrates. 

The  properties  of  the  cryohydrates  of  common  salt  help  to  explain  the 
well-known  fact  that  salt  water  does  not  freeze  except  at  temperatures 
much  below  32°  F.  Salt  water  may  be  considered  as  a  special  chemical 
compound,  the  cryohydrate  of  common  salt.  This  cryohydrate  is  charac- 
terized by  a  melting-point  (and,  what  is  the  same  thing,  a  solidifying- 
point)  which  happens  to  be  below  o°  C. 

Efflorescence.  —  Most  crystals  containing  water  of  crys- 
tallization may  give  it  off  by  mere  influence  of  heating. 
The  amount  of  heat  required  varies  with  the  substances  : 
in  some  cases  the  ordinary  heat  of  the  atmosphere  is 
sufficient.  The  result  of  such  expulsion  is  a  breaking 
of  the  crystals  into  a  non-crystalline  powder.  The  crys- 
tals arexsaid  to  effloresce 


FIFTH  EXAMPLE.  —  Metallic  Lead. 

Melted  lead  is  of  course  a  liquid;  when  it  is  slowly  cooled,  it  permits 
the  formation  of  solid  crystals.  Many  other  melted  metals  and  alloys  do 
the  same,  but  melted  lead  affords  a  good  example,  because  in  many  large 
lead  works  this  crystallization  is  continually  carried  on  on  an  enormous 
scale.  Lead,  as  produced  from  the  ore,  contains  a  minute  amount  of  sil- 
ver diffused  through  it.  By  Pattinson's  process  for  extraction  of  this 
silver  the  melted  mass  is  cooled  and  thus  partly  crystallized;  solid  crystals 
of  nearly  pure  lead  thus  separate,  and  upon  their  removal  they  are  found 
to  have  left  most  of  their  silver  in  the  melted  portion  of  lead  remaining. 
From  this  the  silver  is  finally  extracted. 

Alloys.  —  The  term  alloy  was  originally  applied  to  a 
mixture  of  gold  and  silver  melted  together  with  or  with- 
out other  metals.  The  term  is  now  applicable  to  all 


THE  ATTRACTIONS  OF  MOLECULES. 


129 


mixtures  or  compounds  of  metals  with  each  other,  ex- 
cept those  containing  mercury,  which  latter  are  called 
"amalgams." 

On  melting  two  metals  together,  complete  assimila- 


FIG.  85.  —  Pattinson  furnace  for  separating  crystals  of  pure  lead  (from  its  solution  in 
melted  argentiferous  lead)  by  slow  cooling. 


tion  takes  place  in  some  cases ;   in  others  it  does  not. 
Thus,  silver  does  not  readily  alloy  with  iron. 


FIG.  86.  —  Top  view  of  Pattinson  furnace,  showing  the  kettles  in  which  argentiferous 
lead  is  melted. 


The  physical  properties  of  an  alloy  are,  in  certain 
cases,  the  mean  of  the  properties  of  the  metals  of 
which  it  is  composed ;  in  other  cases  they  are  widely 
different. 


I3O        THE  ATTRACTIONS  OF  MOLECULES. 

Matthiessen  has  divided  the  metals  that  form  alloys  into  two  classes: — 

FIRST.  Those  which  impart  to  their  alloys  their  own  properties :  lead, 
tin,  zinc,  and  cadmium. 

SECOND.     Those  which  do  not :  the  other  metals. 

He  regards  the  alloys  of  class  first  as  solidified  solutions  of  one  metal 
in  the  other.  The  metals  of  class  second  he  considers  enter  into  alloys  in 
allotropic  form. 

(C)    ADHESION  BETWEEN  SOLIDS  AND  GASES.1 

A  solid,  when  immersed  in  a  gas  and  then  withdrawn, 
retains  upon  its  surface  a  thin  film  of  gas,  somewhat  as 
a  solid  is  wetted  by  dipping  in  water. 

Further,  some  solids  absorb  into  their  intermolecular 
spaces  a  great  bulk  of  gas  —  so  much  indeed  that  in  some 
cases  the  absorbed  gas  occupies  a  volume  even  smaller 
than  it  would  if  condensed  to  the  liquid  state  by  itself. 

The  metal  palladium  is  remarkable  for  absorbing  or 
occluding,  at  ordinary  temperature,  eight  hundred  times 
its  bulk  of  hydrogen  gas.  The  late  Professor  Graham, 
of  London,  who  observed  this  property  of  palladium,  con- 
sidered the  solid  thus  formed  to  be  an  alloy,  and  to  con- 
tain hydrogen  in  the  solid  form.  The  quantities  of  the 
two  elements  are  in  this  case  approximately  in  the  pro- 
portion of  the  weight  of  one  atom  of  hydrogen  to  one 
atom  of  palladium,  so  that  it  has  been  suggested  that 
the  substances  may  be  in  chemical  combination. 

The  metal  platinum  has  the  same  power  as  palladium,  though  to  a  less 
degree. 

If  a  warm  piece  of  platinum  foil  is  placed  in  a  current  of  mixed  illu- 
minating gas  and  air,  the  foil  absorbs  portions  of  all  the  gases.  In  so  doing 
it  condenses  them  to  such  a  degree  as  to  bring  the  molecules  very  near  to 
each  other,  even  within  that  minute  distance  through  which  chemical 

1  Section  (Z?)  is  at  p.  114. 


THE  ATTRACTIONS  OF  MOLECULES.        131 

attraction  can  be  exerted.  Chemical  union  does  in  fact  take  place,  as  is 
evidenced  by  the  production  of  light  and  heat  and  other  phenomena  of 
true  combustion. 

Hannay  and  Hogarth  have  shown  that  in  some  cases 
a  gas,  brought  in  contact  with  a  solid,  dissolves  the  latter 
quickly  into  itself. 


FIG.  87.  —  Dbbereiner's  lamp,  showing  the  adhesion  of  hydrogen  gas  to  platinum. 
The  bottom  of  the  lamp  is  a  hydrogen  generator.  Dilute  sulphuric  acid  acts  upon  the 
mass  of  zinc  B.  Hydrogen  rises  in  the  little  bell-glass,  and  streaming  from  the  tip  at  f, 
and  falling  upon  the  mass  of  spongy  platinum  at  G,  takes  fire.  The  burning  hydrogen 
lights  the  oil  lamp  M. 

(D)    ADHESION  BETWEEN  LIQUIDS  AND  LIQUIDS. 

In  general,  the  adhesion  of  liquids  to  liquids  so  far 
exceeds  their  respective  cohesive  forces  that  the  liquids 
may  be  mixed  in  all  proportions. 

In  general,  heat  favors  this  sort  of  diffusion. 

Thus  water  and  ordinary  alcohol,  when  mixed  in  any 
proportion  whatever,  mingle  throughout  by  virtue  of 
their  own  attractive  forces.  On  the  other  hand,  when 
water  and  ordinary  ether  are  mixed,  only  a  certain  small 
amount  of  the  ether  dissolves  in  the  water ;  the  ether  in 


132 


THE    ATTRACTIONS    OF    MOLECULES. 


excess  of  this  amount  forms  a  separate  layer  upon  the 
top  of  the  water. 


FIG.  88.  —  Apparatus  for  demonstrating  the  fact  and  the  amount  of  liquid  diffusion. 
A  given  liquid  is  placed  in  the  vessel  B.  A  solution  to  be  tested  is  placed  in  the  vessel 
A ,  provided  with  a  glass  cover.  At  a  certain  point  of  time  the  cover  of  A  is  removed. 
The  material  in  A  at  once  commences  to  diffuse  into  the  liquid  B.  After  a  proper  period 
of  time  has  elapsed,  the  cover  is  replaced  upon  A .  A  portion  of  the  liquid  B  is  then 
tested,  and  the  amount  of  material  that  has  diffused  from  A  into  B  in  the  given  number 
of  minutes  or  hours  is  determined. 

There  are  many  well-known  examples  of  two  liquids 
which  scarcely  mix  at  all ;  water  and  oil,  water  and 
mercury,  are  such. 


FIG.  89.  —  Graham's  apparatus  for  dialysis. 

Osmose  of  Liquids.  —  In  case  of  two  liquids  separated  by  a  porous 
septum  (it  being  granted  that  there  exists  adhesion  between  the  liquids, 
and  a  difference  in  the  amounts  of  adhesion  of  the  two  liquids  for  the  sep- 


THE  ATTRACTIONS  OF  MOLECULES. 


133 


turn),  the  liquid  which  wets  the  septum  the  better  passes  through  the  more 
rapidly. 


FIG.  90.  —  Dialyzing  apparatus  separated.  The  upper  vessel  is  called  the  dialyzer. 
It  consists  of  a  ring  open  at  top  and  bottom,  the  bottom  opening  being  covered  with  a 
membranous  material,  held  in  place  by  a  stout  rubber  ring. 

Dialysis.  —  The  process  of  dialysis  can  be  displayed  by  means  of  a 
suitable  vessel  divided  by  a  kind  of  membranous  partition  into  two  com- 


FIG.  91.  —  Graham's  apparatus  for  illustrating  dialysis.    A  crystallizable  substance  placed 
in  the  vessel  a,  called  the  dialyzer,  passes  by  liquid  diffusion  into  the  liquid  b. 


partments.     If  pure  water  is  placed  in  one  compartment  and  the  aque- 
ous solution  of  some  crystallizable  substance  in  the  other,  dialysis  takes 


134 


THE  ATTRACTIONS  OF  MOLECULES. 


place;  that  is,  the  crystallizable  substance  makes  its  way  through  the 
diaphragm  into  the  other  compartment.  Non-crystallizable  substances  (for 
this  purpose  called  colloids)  are  not  capable  of  this  kind  of  transfer.  Evi- 
dently the  crystallizable  substances  pass  through  the  diaphragm  by  a  kind 
of  osmose. 

(E)    ADHESION  BETWEEN  LIQUIDS  AND  GASES. 

Water  and  many  other  liquids  have  the  power  of  dis- 
solving gases,  though  in  very  different  proportions. 


FIG.  92.  —  Ammonia  fountain.  The  vessel  A  contains  at  first  ammonia  gas.  As 
water  from  B  passes  up  through  the  little  tube,  the  ammonia  gas  dissolves  so  rapidly  in 
the  water  as  to  produce  diminished  pressure.  Whereupon  the  atmospheric  pressure 
upon  the  surface  of  water  in  B  forces  the  water  into  the  vessel  A  as  in  a  fountain. 


The  amount  of  gas  absorbed  by  a  liquid  upon  which  it 
exerts  no  chemical  action  depends  upon  — 

I.    The  nature  of  the  gas  and  liquid ; 
II.    The    pressure   to   which    they   are    exposed   (the 
amount  of  gas  absorbed  varies  directly  as  the  pressure) ; 


THE  ATTRACTIONS  OF  MOLECULES.        135 

III.  The  temperature  (with  few  exceptions,  tne  solu- 
bility of  a  gas  in  a  liquid  is  greater,  the  lower  the  tem- 
perature). 

Evidences  of  the  difference  in  the  amounts  of  gas 
dissolved  by  a  stated  amount  of  water  may  be  found  in 
the  following  table  :  — 


FIG.  93.  —  Disposition  of  apparatus  for  showing  the  adhesion  of  atmospheric  air  to 
water.  Water  containing  air  is  placed  in  flask  A.  Upon  boiling  this  water  air  is  expelled 
and  some  steam  is  formed.  The  steam  and  air  pass  into  the  bell-glass  C.  The  air  col- 
lects at  the  top  of  the  bell-glass.  The  water-vapor  condenses  on  the  surface  of  the 
mercury. 

TABLE 

SHOWING   AMOUNTS,  BY  VOLUME,  OF  SEVERAL   GASES    SPECIFIED,  DISSOLVED 
BY    IOOO   VOLUMES   OF   WATER   AT   32°  F. 

Amount  of  water  used,  1,000  volumes. 

"         "  hydrogen  gas                                 dissolved,  19  " 

"         "  nitrogen  gas  "  20  " 

"  oxygen  gas  "  41 

"         "  carbon  dioxide  gas  (CO.2)  "  !>796  " 

"          "  hydrosulphuric  acid  gas  (H2S)         "  4»37°  " 

"  sulphur  dioxide  gas  (SO2)  "  68,861  " 

"          "  ammonia  gas  (NHS)  "  1,049,600  " 


136        THE  ATTRACTIONS  OF  MOLECULES. 

The  very  large  amounts  in  several  of  these  cases  are 
believed  to  be  due  to  the  definite  chemical  union  of  the 
gases  with  the  water  to  form  new  compounds. 

It  is  a  fact  worthy  of  mention  that  molten  silver  has  the  power  of  draw- 
ing oxygen  from  the  air  and  dissolving  it  in  a  quantity  equal  to  twenty 


FIG.  94.  —  Apparatus  for  illustrating  diffusion  of  gases.  If  a  heavier  gas  is  placed  in 
the  lower  flask  and  a  lighter  gas  is  placed  in  the  upper  flask,  and  the  stop-cocks  are 
opened,  it  is  found  experimentally  that  the  lighter  gas  diffuses  rapidly  downward  into  the 
other,  and  that  the  heavier  gas  diffuses  upward  (although  more  slowly)  into  the  lighter. 

times  its  own  volume.  When  the  silver  solidifies,  this  gas  is  violently 
expelled.  (The  same  principle  is  manifested  by  water;  upon  freezing,  it 
expels  the  oxygen  and  nitrogen  it  previously  dissolved  from  the  air.) 

(F)   ADHESION  BETWEEN  GASES  AND  GASES. 

The  extraordinary  tendency  of  gases  to  intermingle 
and  interdiffuse  has  already  been  discussed  under  the 


THE  ATTRACTIONS  OF  MOLECULES.       137 

title  of  diffusion  of  gases.   This  tendency  is  so  strong  that 
it  overcomes  the  greatest  differences  of  specific  gravity. 

These  phenomena  are  not  mainly  due  to  adhesion,  how- 
ever, though  there  are  grounds  for  believing  that  there 
is  such  a  thing  as  gaseous  adhesion.  Thus  Regnault 
has  shown  that  when  a  liquid  evaporates  in  the  air, 
more  vapor  rises  than  when  it  evaporates  into  the  same 
volume  of  vacuous  space. 

The  tendency  of   gases  to  intermingle  seems  to   be 
mainly  a   development  of   their   tension  or   expansive 
power.     This  phenomenon  is  due  to  that  motion  within 
the  mass  which  the  molecules  of  all  kinds  of  matter  — 
even  the  most  rigid  —  possess. 

But  the  molecules  of  the  gaseous  form  of  matter  are 
almost  uninfluenced  by  cohesion.  Hence  they  manifest 
this  intermolecular  motion  to  the  most  striking  degree. 
And  so  when  gases  themselves  are  compared,  it  can  be 
proved  that  the  molecules  of  the  lightest  ones  move 
with  the  greatest  rapidity.  Of  course  the  ample  spaces 
between  the  molecules  of  a  gas  offer  great  opportunities 
for  the  entrance  of  the  molecules  of  another  gas. 

The  Terrestrial  Atmosphere.  —  The  atmosphere  of  our 
globe  affords  a  splendid  example  of  gaseous  diffusion 
constantly  at  work  on  a  large  scale. 

I.  The  atmospheric  air  consists  mainly  of  a  mixture 
of  oxygen  gas  and  nitrogen  gas,  in  the  following  propor- 
tions :  — 

COMPOSITION  OF  ATMOSPHERIC  AIR. 

By  volume.  By  weighf. 

Oxygen 20.9  per  cent.  23.1  per  cent. 

Nitrogen 79.1       "  76.9       " 


FIG.  95.  —  Balance  for  showing  that  certain  gases  are  heavier  than  the  atmosphere. 
The  one  jar  contains  atmospheric  air.  When  a  heavier  gas  is  poured  into  the  other  jar, 
the  needle  of  the  balance  is  boldly  deflected. 


THE  ATTRACTIONS  OF  MOLECULES. 


139 


Now  any  given  measure  of  oxygen  gas  is  sixteen  times 
as  heavy  as  the  same  measure  of  the  standard  gas,  hydro- 
gen ;  but  nitrogen  gas  is  only  fourteen  times  as  heavy 
as  hydrogen.  Yet  in  our  atmosphere  the  heavier  oxy- 
gen does  not  settle  out,  but  remains  thor- 
oughly intermingled  with  the  nitrogen. 

2.  The  respiration  of  living  animals 
and  the  burning  of  all  our  chief  fuels 
are  constantly  casting  into  the  atmos- 
phere immense  quantities  of  a  heavy 
gas,  carbon  dioxide  (CO2).  This  gas  is 
twenty-two  times  as  heavy  as  the  stand- 
ard gas,  hydrogen.  Of  course,  therefore, 
it  is  much  heavier  than  the  oxygen  or 
the  nitrogen  of  the  atmospheric  air ;  it 
does  not  settle  out  from  the  air,  how- 
ever, but  promptly  intermingles  with  it 
and  remains  intermingled. 


NOTE  I.     On  the  density  of  atmospheric  air. 

The  air  contains  minute  amounts  of  a  multitude  of 
gases,  but  oxygen  and  nitrogen  so  largely  predominate 
that  only  these  need  be  taken  .into  the  account  here. 

The  density  of  the  air  is  somewhere  between  the 
densities,  1 6  and  14,  of  its  two  chief  constituents  :  it  is 
about  14.4. 


FIG.  96.  —  Reg- 
nault's  method  of 
suspending,  from  the 
balance-pan,  a  globe 
containing  a  gas  to 
be  weighed.  A  globe 
of  similar  volume  is 
also  suspended  from 
the  other  pan. 


I  volume  of  oxygen  gas,  weighing  16  units     .     .     .     .  16.  units. 

4  volumes  of  nitrogen  gas,  each  weighing  14  units.     .  56.     " 

5  volumes  of  mixture  (air)  will  weigh 72.     " 

I  volume  of  air  will  weigh 14.4  " 

NOTE  II.    On  the  density  of  carbon  dioxide  gas  (CO2). 

By  actual  weighing,  in  comparison  with  an  equal  volume  of  the  standard 
gas,  hydrogen,  this  gas  has  been  found  to  have  the  density  22;  i.e.  to 
weigh,  bulk  for  bulk,  22  times  as  much  as  hydrogen. 

The  density  may  be  computed  from  the  molecular  weight  as  follows :  — 


I4O  THE    ATTRACTIONS    OF    MOLECULES, 

Formula  of  a  Molecule  of  Carbon  Dioxide  Gas  (CO%). 

Weight  of  one  atom  of  carbon 12  microcriths. 

"        "  two  atoms  of  oxygen  ( 1 6  X  2)   ...  32          " 

"        "  one  molecule  of  carbon  dioxide      .     .  44          " 
"        "  one  molecule  of  hydrogen,  H2  (i  X  2)       2          " 

Hence  a  molecule  of  carbon  dioxide  weighs  twenty-two  times  as  much 
as  a  molecule  of  hydrogen. 

But  all  gaseous  molecules  have  the  same  size;  hence,  any  volume  of 
carbon  dioxide  weighs  twenty-two  times  as  much  as  the  same  volume 
of  hydrogen. 

NOTE  III.  Of  course,  in  weighing  atmospheric  air  and  other  gases, 
pressure  and  temperature  must  be  considered.  The  pressure  must  be 
measured  by  some  form  of  barometer.  The  temperature  must  be  measured 
by  some  form  of  thermometer. 


CHAPTER    XIII. 
THE  ATTRACTION  OF  ATOMS. 

CHEMICAL  AFFINITY. 

CHEMICAL  affinity  is  an  agency  which  acts  at  in- 
sensibly small  distances,  and  tends  to  produce  combina- 
tions of  certain  atoms  and  molecules  of  matter  into 
groups  of  a  precisely  determinate  kind. 

The  characteristics  of  this  agency  cannot  be  described 
in  a  few  words.  To  it  are  referred  a  multitude  of 
phenomena,  displaying  under  different  circumstances 
the  greatest  variety  of  action.  Such  differences  are  for 
example  :  — 

As  to  the  original  quantity  and  intensity  of  the  activity 
itself. 

As  to  the  conditions  under  which  its  active  powers  are 
displayed. 

As  to  the  methods  by  which  it  works. 

As  to  the  sphere  of  activity  —  extremely  narrow  in  a 
certain  sense  and  extremely  wide  in  another. 

As  to  the  results  accomplished  by  it. 

The  Conditions  favoring  Chemical  Change.  —  I .  This 
force  manifests  its  chief  activity  between  atoms  or  mole- 
cules of  different  kinds. 

Thus,  an  atom  of  hydrogen  has  affinity  for  another 
atom  of  hydrogen,  and  the  two  may  unite  to  form  a 

141 


142  THE    ATTRACTION    OF    ATOMS. 

molecule  of  hydrogen,  expressible  by  H  —  H,  also  written 
Hz.  Again,  an  atom  of  chlorine  has  affinity  for  another 
atom  of  chlorine,  and  these  two  may  unite  to  form  a 
molecule  of  chlorine  expressible  by  the  formula  Cl—  Cl, 
or  C12.  But  when  a  molecule  of  hydrogen  is  brought  in 
contact  with  a  molecule  of  chlorine,  the  two  generally 
surfer  decomposition,  so  that  a  rearrangement  may  take 
place  and  two  new  molecules  of  hydrochloric  acid  (HC1) 
may  be  produced.  This  chemical  change  may  be  ex- 
pressed by  the  following  equation  :  — 

H2  +  C12  2HC1 

One  molecule  of  One  molecule  of  Two  molecules  of 

Hydrogen,  Chlorine,  Hydrochloric  acid, 

2  71  73 

parts  by  weight.  parts  by  weight.  parts  by  weight. 


Evidently  the  atom  of  chlorine  has  more  affinity  for 
an  atom  of  hydrogen  than  for  another  atom  of  chlorine. 
And  an  atom  of  hydrogen  has  more  affinity  for  an  atom 
of  chlorine  than  for  another  atom  of  hydrogen. 

2.  It  is  manifested  between  different  substances  with 
very  different,  though  definite,  degrees  of  force.     Thus 
the  metal  gold  and  the  metal  iron  oxidize  (that  is,  com- 
bine with  oxygen)  with  different  degrees  of  ease  ;  but 
it  is  always  the  iron  that  oxidizes  the  easier. 

3.  Certain  physical  conditions  are  of  great  importance 
in  connection  with   chemical   action.      When    physical 
conditions  are  favorable,  chemical  action  proceeds  with 
great  vigor  ;  when  they  are  unfavorable,  the  same  pro- 
cesses sometimes  fail  to  advance  at  all,  or  they  may  be 
even  reversed.     Under  unfavorable  conditions  chemical 
affinity  appears  either  not  to  exist  or  to  be  dormant. 


THE  ATTRACTION  OF  ATOMS.  143 

The  following  are  some  of  the  physical  conditions  which  determine 
chemical  changes  —  changes  that  may  have  as  their  prominent  features 
either  the  building  up  or  the  breaking  down  of  molecules  :  — 

(rt)  The  Liquid  Condition. — Some  substances  that  chemically 
unite  when  mixed  as  solutions,  manifest  no  affinity  when  they  are  mingled 
in  the  solid  form.  Thus,  solid  tartaric  acid  and  solid  hydro-sodic  carbonate 
when  mingled  manifest  no  change.  When  water  is  added,  however,  each 
solid  dissolves,  and  a  chemical  change  at  once  ensues,  hydro-sodic  tartrate, 
carbon  dioxide,  and  water  being  formed. 

The  chemical  change  may  be  expressed  as  follows :  — 


H404(C4H202) 

One  molecule  of 
Tartaric  acid, 

15° 
parts  by  weight. 

+         2  HNaCO3 

Two  molecules  of 
Hydro-sodic  carbonate, 
1  68 
parts  by  weight. 

2C02          + 

(H2Na2)04(C4H202) 

+          2H20 

Two  molecules  of 

One  molecule  of 

Two  molecules  of 

Carbon  dioxide, 

Hydro-sodic  tartrate, 

Water, 

88 

194 

36 

parts  by  weight. 

parts  by  weight. 

parts  by  weight. 

318 

The  equation  indicates  that  water  is  actually  formed  by  the  operation; 
it  appears  evident,  therefore,  that  the  water  which  acted  as  the  solvent  was 
not  demanded  in  the  building  up  of  the  molecules  produced,  but  did,  in 
fact,  act  as  a  favoring  physical  agent. 

It  appears  to  be  proved,  however,  that  certain  solid  bodies,  finely  pul- 
verized, thoroughly  mixed,  and  then  subjected  to  great  pressure,  produce 
new  compounds  as  the  result  of  the  pressure  (and  not  of  the  heat  attend- 
ant). The  quantities  of  the  compounds  changed  appear  to  increase  with 
the  duration  of  the  pressure  and  its  amount,  as  well  as  the  fineness  and 
thoroughness  of  intermingling  of  the  powders. 

Thus,  in  a  certain  experiment,  mixtures  of  dry,  pure  precipitated  baric 
sulphate  and  sodic  carbonate  were  subjected  to  a  pressure  of  six  thousand 
atmospheres  under  varying  conditions  of  temperature  and  duration  of  the 
pressure.  Afterward  the  product  was  tested.  After  a  single  compression 
the  amount  of  baric  carbonate  produced  was  about  one  per  cent;  the  solid 


144  THE  ATTRACTION  OF  ATOMS. 

block  produced  was  pulverized  and  compressed  again,  when  five  per  cent 
of  barium  carbonate  was  produced;  further  treatment  brought  it  up  to 
eleven  per  cent. 

It  has  been  concluded  that  — 

1.  A  sort  of  diffusion  takes  place  in  solid  bodies. 

2.  Matter  assumes  under  pressure  a  condition  relative  to  the  volume  it 
is  obliged  to  occupy. 

3.  For  the  solid  state,  as  for  the  gaseous,  there  is  a  critical  tempera- 
ture above  or  below  which   changes  by  simple  pressure   are  no  longer 
possible. 

(£)  Heat.  —  Many  substances,  when  practically  in  contact  with  each 
other,  do  not  combine  chemically  unless  the  whole  or  a  portion  of  the 
mass  is  raised  to  some  definite  point  of  temperature.  When  this  point  is 
reached,  union  at  once  commences. 

The  process  of  combustion  of  ordinary  fuels  affords  an  appropriate  illus- 
tration. If  a  portion  of  a  mass  of  coal  is  heated  in  the  air  to  the  point  at 
which  union  with  oxygen  takes  place,  the  phenomena  of  combustion  (a 
form  of  chemical  union)  are  witnessed.  The  chemical  change  initiated 
may  be  expressed  in  part  as  follows :  — 

c  +  o,  co2 

One  atom  of  One  molecule  of  One  molecule  of 

Carbon,  Oxygen,  Carbon  dioxide, 

12  32  44 

parts  by  weight.  parts  by  weight.  parts  by  weight. 

44  44 

It  is  an  interesting  fact  that  generally  the  combustion  of  the  first  por- 
tions of  the  coal  evolve,  by  the  act  of  chemical  union,  sufficient  heat  to 
raise  yet  other  portions  to  the  igniting  point.  This  process,  repeated,  ena- 
bles the  operation  to  proceed  from  portion  to  portion  so  long  as  the  supply 
of  carbon  and  oxygen  are  kept  up  —  unless,  indeed,  some  unfavorable 
physical  condition  is  allowed  to  supervene. 

There  are  numerous  other  examples  known,  in  which  chemical  action  is 
stimulated  by  an  amount  of  heat  insufficient  to  produce  light. 

In  fact,  addition  of  heat  is  the  method  oftenest  used  for  developing  or 
arousing  chemical  affinity. 

Thermolysis  and  Dissociation.  —  Another,  and  at  first  seemingly 
inconsistent  chemical  effect  of  heat,  ought  to  be  mentioned  here.  It  has 


THE    ATTRACTION    OF    ATOMS. 


145 


already  been  pointed  out  that  addition  of  heat  expands  material  bodies, 
and  even  changes  solids  and  liquids  to  the  gaseous  form.  (See  p.  43.) 
These  effects  are  believed  to  be  essentially  associated  with  a  motion  of  the 
particles  of  the  body,  such  that  the  molecules  are  moved  farther  and  far- 
ther apart,  and  even  beyond  the  range  of  influence  of  those  cohesive  forces 


FIG.  97.  —  Henri  St.  Clair  Deville,  distinguished  French  chemist,  noted  for  his  discoveries 
in  the  chemistry  of  high  temperatures,  dissociation,  for  example. 

that  bind  them  into  solid  and  liquid  masses.  It  would  be  quite  consistent 
with  this  view  if  still  greater  addition  of  heat  were  found  to  be  sufficient  to 
drive  even  atoms  apart  from  each  other,  and  so  to  place  them  beyond  the 
minute  distances  within  which  the  force  of  chemical  affinity  is  exerted. 
This  would  result  in  a  decomposition  of  compound  molecules  and  a  lessen- 


146  THE  ATTRACTION  OF  ATOMS. 

ing  of  the  number  of  atoms  capable  of  existing  together  in  elementary 
molecules. 

Now  the  experiments  of  Deville  and  others  fully  confirm  these  sugges- 
tions. It  is,  in  fact,  proved  that  certain  substances,  as  water,  for  example, 
may  be  decomposed  into  their  elements  by  influence  of  high  temperature 
alone.  In  this,  and  some  similar  cases,  the  elements  may  reunite  when  the 
temperature  of  the  mixture  falls  slightly.  This  kind  of  temporary  decom- 
position is  called  dissociation.  It  may  be  added  that  light  and  electricity, 
as  well  as  heat,  are  in  some  cases  capable  of  accomplishing  it. 

In  another  class  of  cases,  of  which  ammonia  gas  (NH3)  may  serve  as 
an  example,  the  molecule  '^permanently  broken  up;  that  is,  its  elementary 
substances  do  not,  by  fall  of  temperature,  rejoin  to  produce  the  original 
compound.  In  such  cases  the  operation  is  called  thermolysis. 

It  is  also  observed  that  certain  elementary  substances,  as  sulphur,  for 
example,  manifest  a  gradual  lessening  of  their  relative  vapor  densities  as  they 
are  raised  to  higher  and  higher  temperatures.  This  lessening  of  vapor 
density  is  accepted  as  an  indication  that  the  molecules  contain  fewer  and 
fewer  atoms;  that  is,  undergo  dissociation.  The  methods  of  Victor  Meyer 
and  others  have  directed  attention  to  this  subject. 

Heat  often  produces  a  modification  of  the  relative  chemical  attractions 
of  bodies.  Thus,  at  ordinary  temperatures,  sulphuric  acid  is  capable  of 
displacing  boric  acid  from  its  salts  in  solutions.  At  high  temperatures  — 
the  red  heat,  for  example  —  the  chemical  affinities  are  reversed  :  boric  acid 
displaces  sulphuric  acid. 

In  a  few  cases  chemical  decomposition  is  producible  by  mechanical 
means,  as,  for  example,  in  certain  explosive  compounds;  but  it  is  proba- 
ble that  the  mechanical  is  not  always  the  immediate  cause.  In  the  familiar 
cases  where  mechanical  percussion  produces  decomposition  of  certain 
explosives,  evidently  the  heat  generated  by  the  percussion  is  the  true 


(<:)  Light.  —  This  agent,  as  usually  produced  by  luminous  bodies,  is 
by  no  means  a  homogeneous  one;  the  prism  shows  it  to  be  divisible  into 
thousands  of  kinds  of  energy,  characterized  by  greater  or  less  differences. 
The  white  light,  as  emitted  by  most  of  its  sources,  has  at  least  three  classes 
of  rays,  —  luminous  rays  of  various  colors,  non-luminous  chemical  rays,  non- 
luminous  heat  rays.  The  non-luminous  chemical  rays,  called  also  actinic 
rays,  have  a  specific  power  of  determining  the  chemical  union  of  certain 
elements  and  the  chemical  decomposition  of  certain  compounds. 

Thus  chlorine  gas  and  hydrogen  gas,  when  mixed  in  a  dark  room,  do 


THE    ATTRACTION    OF    ATOMS. 


147 


not  readily  unite;    when  such  a  mixture   is  exposed  to  sunlight,  almost 
instantaneous  combination  ensues. 

The  decomposing  influence  of  certain  rays  of  light  is  displayed  in  the 
photographic  print,  the  substance  decomposed  being  argentic  chloride. 

(</)  Electricity.  —  The  influence  of  electricity  in  connection  with 
chemical  action  is  manifested  in  at  least  four  different  forms. 

FIRST.     Operations  coming  under  the  head  of  electrolysis. 

Electricity  of  low  tension,  such  as  that  produced  by  the  galvanic  bat- 
tery, is  capable  of  most  important  influences  on  chemical  compounds.  In 


FIG.  98.  —  Apparatus  showing  how  an  electric  current  may  precipitate  a  metal 
from  its  solution. 


processes  of  electro-plating  with  copper,  nickel,  silver,  gold,  and  other 
metals,  it  sets  in  motion  an  invisible  current  by  which  atoms  of  metal  are 
driven  away  from  the  metallic  plate  called  the  anode,  then  into  the  mole- 
cules of  acid  or  metallic  salt  dissolved  in  the  plating-bath,  and  thence  upon 
the  surface  of  the  object  to  be  plated,  called  the  cathode.  The  deposition 
of  the  metal  is  merely  incidental;  the  molecular  transfer  is  the  important 
feature.  Metals  may  be  dissolved  by  this  method  if  desired.  Again,  non- 
metals  may  be  made  to  combine,  or,  if  combined,  may  be  separated  in  like 
fashion  by  the  current. 

SECOND.  When  a  current,  in  the  form  of  an  electric  arc,  passes  through 
compounds  or  through  mixtures  of  elementary  substances,  chemical  changes 
often  occur.  One  of  the  most  marked  illustrations  of  this  kind  of  influ- 


148 


THE    ATTRACTION    OF    ATOMS. 


ence  is  in  the  direct  union  of  carbon  and  hydrogen  whereby  the  substance 
known  as  acetylene  (C2H2)  is  formed.  One  of  the  most  important  features 
of  interest  in  connection  with  this  operation  is  the  fact  that  acetylene  rep- 
resents a  starting-point  for  the  synthesis,  or  building  up,  of  organic  com- 
pounds directly  from  their  elements. 

THIRD.  Another  form  of  action  is  by  the  influence  of  the  electric  spark 
from  a  Ruhmkorff  coil,  a  Holtz  machine,  or  similar  appliance.  In  this  way 
distinct  chemical  action  is  stimulated  over  a  limited  field.  The  field,  how- 
ever, may  be  widened  by  continuance  of  the  electric  discharge. 


FIG.  99.  —  Apparatus  for  decomposing  water  into  its  components,  hydrogen  and 
oxygen,  by  means  of  a  galvanic  current  generated  by  a  Bunsen  battery. 

Such  an  electric  discharge,  in  the  form  of  sparks  flowing  from  platinum 
terminals  through  dry  atmospheric  air,  gives  rise  to  a  direct  union  of  the 
oxygen  and  nitrogen.  Brown  fumes  of  N.2O4  or  NO2  are  thus  formed. 
These  with  water  may  form  nitric  acid  (HNO3). 

Probably  lightning  discharges  form  nitric  acid,  in  this  way,  and  thus 
contribute  to  the  available  nitrogen  of  the  soil. 

FOURTH.  The  silent  electric  discharge  produces  certain  marked  effects, 
of  which  the  most  noteworthy  is  the  change  of  ordinary  oxygen  into  ozone. 

In  all  these  cases  the  action  may  be  twofold.  There  is  the  true  electric 
influence,  and,  especially  in  cases  of  the  second  and  third  methods  already 
referred  to,  there  is  the  additional  influence  of  the  heat  connected  with  the 
arc  or  with  the  luminous  discharge. 


g  S 

l! 
*• 


'35    »• 


150 


THE    ATTRACTION    OF    ATOMS. 


(<?)  Vital  Processes  of  the  Higher  and  Lower  Living  Beings.  — 

The  vital  powers  of  the  higher  orders  of  animal  and  vegetable  beings  have 
most  marked  influence  upon  chemical  action.  Thus  vast  numbers  of  com- 
pounds are  recognized  as  existing  in  living  animals  and  plants  that  have 
not  yet  been  produced  without  the  intervention  of  vital  force.  A  few  crys- 


-^ — •-J^  / 

FlG.  ioi.  —  Ruhmkorff,  the  celebrated  manufacturer  of  electric  instruments,  and 
inventor  or  the  Ruhmkorff  coil. 

tallizable  substances,  ordinarily  the  products  of  living  organisms,  have  lately 
been  produced  by  circuitous  chemical  operations  without  intervention  of 
life.  Doubtless  others  will  be  in  future. 

Certain  processes  of  acetic,  butyric,  and  other  fermentations,  purely 
chemical  in  their  nature,  have  been  shown  to  be  due  to  the  presence  and 
action  of  microbes,  and  not  to  go  on  in  their  absence. 


THE    ATTRACTION    OF    ATOMS.  15! 

Organic  and  Inorganic  Compounds.  —  Chemists  long  ago  recog- 
nized certain  differences  between  the  substances  found  in  distinctly  animal 
and  vegetable  matters,  on  the  one  hand,  and  the  substances  found  in  min- 
eral matters,  on  the  other  —  between  those  things  which  constitute  organ- 
isms like  animals  and  plants,  as  opposed  to  non-living  substances  like  clay, 
iron-rust,  alum,  saltpetre,  etc. 

Animal  matters  and  vegetable  matters  are  the  products  of  bodies  pos- 
sessing organs.  Organs  are  parts  having  specific  functions.  Thus  the 
stomach  is  an  organ  possessing  the  function  of  digestion,  and  the  lungs  are 


FIG.  102. — The  Bunsen  battery,  F,  energizes  the  Ruhmkorff  coil,  E,  affording  a 
series  of  electric  sparks  in  the  flask  A.  The  flask  contains  a  mixture  of  nitrogen  and 
oxygen  in  a  very  dry  condition.  The  sparks  lead  the  two  gases  to  combine. 


organs  possessing  the  function  of  respiration.  Again,  the  leaves,  the 
flowers,  the  seeds,  the  roots,  of  plants,  are  separate  organs,  and  they 
possess  special  and  very  different  functions  of  the  living  vegetable  to  which 
they  belong.  Accordingly,  substances  derived  from  vegetables  and  animals 
are  called  organic.  Non-living  objects,  as  rocks  and  other  mineral  and 
earthy  substances,  do  not  possess  organs,  and  they  have  long  been  called 
inorganic. 

This  division  of  matters  into  organic  and  inorganic  was  formerly  thought 
an  essential  one ;  it  is  not  now  considered  so.  It  is  now  known  that  the 
chemical  changes  of  living  animals  and  plants  are  governed  by  the  same  laws 
as  those  prevailing  in  the  changes  of  rocks  and  other  lifeless  forms  of  matter. 


152 


THE  ATTRACTION  OF  ATOMS. 


Grounds  for  this  Division. —  Chemistry  is  still,  however,  commonly 
divided  into  the  two  great  departments,  —  inorganic  chemistry  and  organic 
chemistry;  but  this  division  is  recognized  as  a  matter  of  convenience 
mainly. 


FIG.  103.  —  Yeast  plant,  illustrating  the  formation  of  additional  cells  by  fission. 


Three  reasons,  which  may  be  mentioned,  why  the  distinction  is  still 
maintained,  are :  — 

FIRST.     The  number  of  organic  compounds  is  very  great. 


FIG.  104.  —  Globules  of  wheat  starch,  as  FIG.  105.  —  Globules  of  potato  starch, 

seen  under  the  microscope,  showing  cellular       as   seen   under  the    microscope,   showing 
structure.  cellular  structure. 

SECOND.  These  compounds  perform  varied  and  important  offices  in 
connection  with  human  beings  in  their  growth  and  nourishment  in  health, 
and  in  their  treatment  in  illness. 

THIRD.     The  processes  of  analysis  and  the  methods  of  investigation  in 


THE  ATTRACTION  OF  ATOMS. 


153 


Organic    compounds   are  slightly  different,   as   a  whole,   from   those   that 
serve  for  the  study  of  inorganic. 

Definition  of  Organic  Chemistry.  —  The  inorganic  and  the  or- 
ganic worlds  are,  however,  so  closely  allied  in  some  respects,  and  certain 
of  the  substances  of  the  one  have  such  close  and  natural  affiliations 
with  those  of  the  other,  that  it  is  often  found  difficult  to  determine 
where  shall  be  placed  the  line  of  demarcation  between  these  two  great 
natural  groups.  In  fact,  chemists  have  not  found  the  definition  incidentally 
introduced  in  a  preceding  paragraph  sufficiently  distinct.  To  make  it 
more  so,  organic  chemistry  has  been  sometimes  called  the  chemistry  of  the 
carbon  compounds.  It  has  sometimes  been  called  the  chemistry  of  the 


FIG.  106.  —  Globules  of  corn  starch  as  seen  under  the  microscope,  showing 
cellular  structure. 


hydrocarbons.  Again,  the  following  still  more  rigid  and  scientific  state- 
ment is  often  employed :  organic  chemistry  includes  those  compounds  in 
which  the  atoms  of  carbon  are  directly  united  either  with  other  atoms  of 
carbon,  or  with  atoms  of  hydrogen,  or  with  atoms  of  nitrogen  1 

Two  Classes  of  Organic  Compounds. — There  is  one  distinction 
between  the  classes  of  organic  compounds  themselves  that  ought  not  to  be 
omitted  here.  The  members  of  the  organic  family  differ  very  much  in 
their  properties,  according  as  they  are  crystalline  or  cellular.  Crystalline 
organic  compounds,  of  which  cane  sugar  may  be  taken  as  a  familiar  and 
suitable  example,  are  numerous.  These  compounds  are  closely  allied  in 
some  respects  to  inorganic  compounds.  They  do  not  seem  to  have  so 


1  From  the  Author's  work,  "  Beginner's  Handbook  of  Chemistry." 


154  THE    ATTRACTION    OF    ATOMS. 

close  a  relation  to  the  vital  processes  as  might  at  first  be  supposed.  But 
those  organic  compounds  that  are  cellular,  such,  for  example,  as  the 
different  varieties  of  starch,  the  fibre  of  wood,  and  the  fibre  of  lean  meat, 
are  much  removed  from  inorganic  bodies,  and  seem  to  bear  a  peculiar  and 
close  relation  to  the  vital  forces.  In  general,  cellular  organic  compounds 
are  called  organized ;  while  the  non-cellular  organic  compounds  are  called 
non-orga  nized. 

Compound  bodies  then  are  divided  from  a  certain  point  of  view  into 
two  great  classes,  —  inorganic  and  organic.  The  organic  are  again  divided 
into  two  classes,  —  organized  and  non-organized. 


CHAPTER    XIV. 

THE  ATTRACTION  OF  ATOMS   (continued). 
THE  CHEMICAL  WORK   OF   MICRO-ORGANISMS. 

IT  is  difficult  to  form  a  proper  conception  of  the  vast 
amount  of   chemical    work  accomplished   by  those  ex- 


I/M 


FIG.  107.  —  Autograph  letter  of  Louis  Pasteur,  being  an  order  for  board  for  hydrophobia 
patients  undergoing  treatment  at  the  Pasteur  Institute. 

ceedingly  minute  parasitic  plants  called  micro-organisms, 
microbes,  bacteria,  etc.  Of  late  years,  following  the 
work  of  Pasteur  and  Koch,  many  observers  have  indus- 


56 


THE    ATTRACTION    OF    ATOMS. 


triously  studied  this  subject.  As  a  result,  microbes  or 
bacteria  (the  word  bacterium  is  used  in  a  general  as  well 
as  in  a  special  sense)  are  now  recognized  as  of  high  im- 
portance in  chemistry. 


FIG.  108.  — Dr.  Robert  Koch,  celebrated  investigator  In  the  field  of  bacteriology. 

These  organisms  are  exceedingly  numerous  as  varieties 
and  yet  more  as  individuals.  They  are  most  widely  dif- 
fused. They  exist  in  air  (though  not  largely  in  sea  air 
nor  even  in  the  air  of  large  sewers),  in  water  (though  in 


THE  ATTRACTION  OF  ATOMS.  157 

varying    quantities  and    kinds),  in   the  soil    (though  in 
most  cases,  not  at  great  depths). 

They  effect  many  kinds  of  decomposition  of  mole- 
cules —  a  work  essentially  chemical.  Some  of  it  is  of 
industrial  interest  in  manufactures  and  in  agriculture ; 
some  of  it  leads  to  the  wonderful  fermentative  and 
putrefactive  processes  of  the  world ;  some  of  it  goes  on 
as  the  chief  factor  in  diseases  of  the  higher  animals,  and 
indeed  in  the  normal  digestive  operations  of  them. 


FIG.  109.  —  Microscopic  infusoria  such  as  are  found  in  stagnant  natural  water. 

(Thus  it  is  observed  that  not  all  microbes  are  pathog- 
enic :  some  are  distinctly  beneficial  to  living  animals.) 

General  Description  of  Microbes.  —  The  organisms  in  question 
consist  of  very  minute  cells — often  not  greater  in  length  than  one  ten- 
thousandth  of  an  inch. 

Yet,  when  floating  in  the  air,  they  cannot  pass  through  a  small  plug  of 
loose  cotton  fibres. 

In  form  they  vary  very  much.  The  common  forms  are  the  globular 
(micrococcus),  the  keyhole-shaped  (like  two  spheres  in  contact,  or  partly 
run  together) ,  the  rod-shaped  (bacillus  form),  the  comma-shaped,  the  spiral 
shaped. 


158 


THE    ATTRACTION    OF    ATOMS. 


The  cells  are  often  grouped  in  a  tolerably  definite  way  in  filaments  01 
chains;  sometimes  they  are  gathered  in  great  irregular  masses. 

A  given  cell  usually  consists  of  a  sac  of  mycoprotein  enclosing  homo- 
geneous protoplasm. 


FIG.  no.  —  aa,  mycoderma  vini;  bb,  mycoderma  aceti  (earlier  stage  of  development); 
cc,  mycoderma  aceti  (advanced  stage  of  development) . 


Classification  and  Nomenclature  of  Bacteria.  —  Micro-organ- 
isms have  been  divided  into  two  sections, —  (i)  the  Endosporea  and  (2) 
the  Arthrosporea, 

The  former  consists  of  but  one  genus,  sporobacterium,  which  has  four 
recognized  species. 

The  latter  (Arthrosporea)  has  two  genera,  bacterium  and  micrococcus, 


THE  ATTRACTION  OF  ATOMS.  159 

The  genus  bacterium  is  the  more  numerous,  having  at  least  twenty-five 
distinct  species,  among  which  are  the  bacteria  found  in  the  human  body 
in  the  diseases  of  consumption,  pneumonia,  cholera,  diarrhoea,  typhoid 
fever,  and  glanders;  also  the  forms  which  are  found  in  foul  ponds  and 
sewage :  it  also  includes  the  vinegar  ferment,  which  converts  ethylic 
alcohol  into  acetic  acid. 


FIG.  in.  —  Mycoderma  aceti,  or  mother  of  vinegar,  as  seen  (enlarged  500  diameters) 
under  the  microscope. 

The  genus  micrococcus  has  eight  species  now  enumerated,  among  which 
are  the  bacteria  of  small-pox,  erysipelas,  scarlet  fever,  and  others. 

Growth  of  Microbes.  —  The  cells  of  micro-organisms 
are  capable  of  extremely  rapid  multiplication  —  generally 


i6o 


THE    ATTRACTION    OF    ATOMS. 


by  fission  or  some  modification  of  it.  Sometimes  fission 
takes  a  course  whereby  forms  like  spores  are  produced. 
By  such  processes  a  single  bacterium  cell  may  multiply 
in  twenty-four  hours  to  more  than  a  billion  individuals 
like  itself. 


'1'1    ""'"    '      '  :  "     '  .'  '   •  "'  ' 


FIG.  112.  —  Apparatus  used  in  Pasteur's  laboratory  in  Paris;  boiler  (sterilizer)  heated 
by  gas,  for  destroying  microbes  by  steam  under  high  pressure;  oven  for  culture  of  certain 
microbes  at  a  definite  temperature;  oven  for  sterilizing  tubes  and  flasks  by  hot  air. 

The  number  of  microbes  in  some  kinds  of  food  is  very 
great.  Thus,  it  has  been  computed  that  in  the  case  of 
certain  kinds  of  Swiss  cheese  one  pound  of  the  article 
possesses  a  microbian  population  greater  than  the  human 
population  of  the  terrestrial  globe, 


FIG.  113.  —  Large  oven  used  in  Pasteur's  laboratory  for  the  culture  of  certain  selected 
microbes.     (It  is  provided  with  an  automatic  gas  regulator.) 


162 


THE  ATTRACTION  OF  ATOMS. 


Micro-organisms  are  influenced  in  their  growth  by  prevailing  conditions; 
some  conditions  are  highly  favorable,  some  unfavorable. 

I.  They  need  a  certain  temperatiire,  varying  in  particular  cases  (100° 
F.  is  generally  favorable).  They  are  best  killed  by  very  high  temperatures 


FIG.  114. —  Sterilizing  apparatus.  It  is  for  the  purpose  of  destroying  bacteria  in  such 
substances  as  may  be  placed  in  the  flasks.  The  apparatus  consists  essentially  of  a  strong 
vessel,  made  tight,  and  provided  with  a  safety  valve  and  steam  gauge.  It  has  jackets  to 
prevent  loss  of  heat  and  condensation  of  steam.  Upon  applying  a  strong  gas  flame 
underneath  the  vessel,  the  water  in  the  vessel  is  raised  to  the  boiling-point.  High  pres- 
sure steam  is  produced.  The  flasks  are  therefore  subjected  to  a  temperature  sufficient  to 
destroy  any  microbes  in  them.  The  flasks  are  then  withdrawn  all  at  once  by  means  of 
the  caster.  The  plugs  of  sterilized  cotton  in  the  necks  of  the  flasks  prevent  subsequent 
access  of  microbes  from  the  air. 


(212°  F.  and  upward).     Some,  however,  succumb  at  even  moderately  low 
temperatures  (50°  F.  and  downward). 


THE    ATTRACTION    OF    ATOMS. 


163 


2.  They  flourish  best  in  presence  of  moisture.  A  small  amount  will 
serve.  Moderately  dry  dust  containing  them,  when  put  under  proper  con- 
ditions with  moisture,  shows  life  by  the  multiplication  of  the  varieties 
present.  Bui-  thorough  desiccation  is  fatal. 


FIG.  115.  —  Louis  Pasteur,  celebrated  for  his  studies  in  the  diseases  produced  by 
micro-organisms. 

3.  Some  of  them  need  atmospheric  air;    some  flourish  best  in  absence 
of  it. 

As  bacteria  are  destitute  of  chlorophyll,  they  do  not  obtain  nutrition  by 
decomposing  carbon  dioxide  of  the  air  under  influence  of  sunlight. 

4.  They  must  have  suitable  pabulum.     Farinaceous  matters  are  good; 
albuminoid    or  other  nitrogeneous  substances  are  very  favorable;    meat 
extract  is  excellent. 


THE    ATTRACTION    OF    ATOMS. 


Some  bacteria  attack  only  dead  organized  tissues;    others  attack  and 
disorganize  tissues  of  living  beings. 

Some  obtain    nitrogen    from  as  simple    compounds   as    ammonia   gas 


a  l.irge 
soi  iltfer 


i]  •  n 


('  i:  i  c  rc 


•tHi 


1:1 1 


Ci tm- 


1 66  THE    ATTRACTION    OF    ATOMS. 

enable  them  to  produce,  in  the  aggregate,  great  quanti- 
ties of  such  compounds  as  they  are  able  to  form. 

The  general  tendencies  of  bacterial  growth  involve  a 
breaking-down  of  complex  molecules  into  somewhat  sim- 
pler ones,  although  this  is  not  the  invariable  result. 

Microbes  produce  certain  special  compounds  as  has 
already  been  suggested. 

1.  They  accomplish  the  following  important  transformations :  — 
Cane  sugar  to  ethyl  alcohol ; 

Glycerin  to  ethyl  alcohol,  and  thence  to  butyl  alcohol ; 

Cane  sugar  to  gum  or  mannite  ; 

Grape  sugar  or  milk  sugar  or  glycerin  to  lactic  acid  and  butyric  acid; 

Urea  to  ammonic  carbonate  ; 

Hippuric  acid  to  benzoic  acid ; 

Albumens  to  ptomaines  ; 

Nitrogenous  matters  to  nitrates. 

2.  They  produce   certain   groups  of  substances   possessed  of  general 
properties,  of  which  the  following  may  be  noted :  — 

Substances  having  marked  agreeable  or  disagreeable  odors; 

Substances  having  brilliant  colors; 

Substances  —  called,  in  general,  ptomaines  —  having  eminently  poison- 
ous properties,  as  tyrotoxicon  in  milk  and  cheese. 

The  ptomaines  just  referred  to  are  alkaloids  of  a  highly  poisonous  char- 
acter, generally  resulting  from  a  morbid  decomposition  of  albuminoids  under 
the  influence  of  microbes. 

The  leucomaines  are  analogous  poisonous  alkaloids,  but  they  are  pro- 
duced by  the  ordinary  physiological  processes  of  the  higher  animals,  and 
thus  are  capable  of  being  decomposed  and  excreted  under  the  normal 
action  of  the  appropriate  organs,  of  which,  apparently,  the  liver  is  the 
most  effective. 

It  was  formerly  held  that  the  morbid  conditions  recognized  in  animals 
affected  by  certain  contagious  and  infectious  diseases  were  due  directly  to 
the  specific  microbes  present.  At  present  the  abnormal  action  of  the  organ- 
ism is  referred  rather  to  the  poisonous  ptomaines  produced  by  the  microbes. 

Usefulness  of  Bacteria  in  the  Organic  World.  —  One  of 
the  most  marked  features  in  the  life-processes  of  the 


THE  ATTRACTION  OF  ATOMS.  l6/ 

higher  animals  and  plants  is  the  circulation  of  certain 
atoms.  That  is,  there  seems  to  be  a  definite  and  rather 
small  stock  of  certain  useful  elements,  like  nitrogen, 
phosphorus  (and  to  these  may  be  added,  with  less  force, 
potassium  and  even  carbon),  which  are  in  a  continual 
state  of  transfer.  This  "  stock  "  is  absorbed  from  the 
soil  by  living  plants ;  it  is  then  absorbed  by  living 
animals.  The  bacteria  assist  the  process  of  animal 
digestion,  whereby  the  vegetable  molecules  are  altered. 
Upon  the  death  of  animals  the  current  stock  returns  to 
the  soil,  thence  to  be  employed  by  a  new  set  of  growing 
plants,  and  later  by  a  new  population  of  living  animals. 
Without  microbes  the  "stock"  would  be  withdrawn 
from  circulation  in  living  animals  or  vegetables,  and 
locked  up  inactive  in  dead  bodies.  Upon  the  death  of 
the  animal,  the  microbes  set  up  those  processes  of 
putrefaction  and  decay,  whereby  the  stable  molecules  in 
the  dead  bodies  become  available  for  the  food  of  grow- 
ing plants. 


CHAPTER    XV. 

THE  ATTRACTION  OF  ATOMS   (continued}. 
MODES   OF  CHEMICAL  ACTION. 

As  a  result   of   the    operation    of   chemical  affinity, 
molecules  are  changed  in  a  variety  of  ways. 

The  following  are  some  of  the  principal  ones  :  — 

I.    Elementary  or  compound  molecules  may  directly 
combine :  — 


Zn 

r-                       Ci2 

ZnCl2 

One  atom  of 

One  molecule  of 

One  molecule  of 

Zinc, 

Chlorine, 

Zinc  chloride, 

65 

71 

136 

parts  by  weight. 

parts  by  weight. 

parts  by  weight. 

J36  136 

2.  An  element  or  group  of  elements  may  displace 
another  element  or  group  :  — 

2HC1  +  Zn  ZnCl,          +          H2 

Two  molecules  of  One  atom  of  One  molecule  of  One  molecule  of 

Hydrochloric  acid,  Zinc,  Zinc  chloride,  Hydrogen, 

73  65  136  2 

parts  by  weight.  parts  by  weight.  parts  by  weight.  parts  by  weight. 

138  138 

But  in  some  cases  the  displacement  may  be  by  gradual 
stages.  Thus  marsh  gas  (CH4)  may  have  its  hydrogen 
replaced  by  chlorine,  atom  by  atom,  until  all  is  removed. 

168 


THE  ATTRACTION  OF  ATOMS.  169 

Thus  the  following    compounds    may  be    progressively 
formed  :  — 

CH3C1, 

CH2C12, 

CHC13, 

CC14. 

3.  An  element  or  group  of  elements  in  one  molecule 
may  exchange  places  with  an  element  or  group  of  ele- 
ments in  another  molecule  :  — 


CuSO4      + 

Ba(NO,)a 

BaSO4 

f        Cu(N03)2 

One  molecule  of 

One  molecule  of 

One  molecule  of 

One  molecule  of 

Cupric  sulphate, 

Baric  nitrate, 

Baric  sulphate, 

Cupric  nitrate, 

159 

261 

233 

187 

parts  by  weight. 

parts  by  weight. 

parts  by  weight. 

parts  by  weight. 

420  420 

4.  There  may  be   a   rearrangement    of   elements    or 
groups  of  elements  within  single  molecules  of  a  sub- 
stance :  — 

(NH4)O(CN)     changes  spontaneously  into     N2H4(CO) 

One  molecule  of  One  molecule  of 
Ammonic  cyanate,  Urea, 

60  60 

parts  by  weight.  parts  by  weight. 

~6o  60 

5.  There  may  be  a  direct  decomposition  of  a  certain 
molecule  into  others  of  a  different  kind  :  — 

2  H.,O  may  be  decomposed  into  2  H2  +  O2 

Two  molecules  of  Two  molecules  of  One  molecule  of 

Water,  Hydrogen,  Oxygen, 

36  4  32 

parts  by  weight.  parts  by  weight.  parts  by  weight. 


I/O  THE  ATTRACTION  OF  ATOMS. 

The  Sphere  of  Chemical  Action.  —  The  sphere  of 
chemical  action  is  evidently  that  of  the  individual  mole- 
cule ;  as  a  result  of  chemical  change,  molecules  change 
their  components.  This  sphere  is  a  very  limited  one 
when  looked  at  with  reference  to  the  minuteness  of  a 
single  molecule.  It  is  one  of  very  wide  range  when  it 
is  remembered  that  all  material  substances  are  made  up 
of  molecules,  and  that  the  character  of  the  molecules 
determines  the  character  of  the  mass.  Chemical  change, 
therefore,  is  most  fundamental,  altering  substances  in 
their  ultimate  recesses.  Changing  the  molecules  in 
which  the  identity  of  substances  reside,  it  changes  the 
identity  of  masses  themselves.  Thus  all  the  kingdoms 
of  nature  owe  to  chemical  action  the  variety  of  sub- 
stances produced  in  their  normal  or  abnormal  growth, 
while  geologic  and  cosmic  changes  involve  chemical 
action  and  reaction  on  the  largest  scale. 

The  Results  of  Chemical  Action.  —  The  effects  pro- 
duced by  chemical  change  are  recognized  as  of  the  most 
striking  kind ;  and  this  is  true  both  in  natural  and  in 
artificial  processes.  t 

Some  of  the  principal  effects  noticed  are  changes  of 
physical  condition,  as  a  substance  originally  solid  or 
liquid  or  gaseous,  at  a  given  temperature,  may  change 
to  another  of  these  conditions  ;  changes  of  color,  odor, 
taste,  or  other  physiologic  or  toxic  effect ;  change  of 
volume:  thus  sometimes  chemical  action  draws  atoms 
closer  together.  As  already  stated  (p.  78),  two  volumes 
of  hydrogen  gas  and  one  volume  of  oxygen  gas,  when 
chemically  actuated,  unite  to  form  a  new  substance 
(water- vapor),  occupying  only  two  volumes  altogether. 


THE    ATTRACTION    OF    ATOMS.  I/I 

Sometimes  there  is  no  reduction,  but  rather  expansion  : 
thus  gunpowder,  a  solid,  experiences  chemical  change 
when  slightly  heated,  and  produces  an  immense  volume 
of  gas.  Sometimes  neither  expansion  nor  contraction 
takes  place. 

Sometimes  chemical  affinity  produces  such  violent  or 
bizarre  effects  that  there  can  be  no  question  that  it  is  in 
active  exercise.  In  other  cases,  where  two  or  more  sub- 
stances might  be  supposed  to  undergo  chemical  change, 
the  evidences  are  so  slight  as  to  make  the  very  exist- 
ence of  the  chemical  action  difficult  to  substantiate. 

Among  all  these  various,  and  in  many  cases  inexplica- 
ble results,  two  principles  are  constantly  recognized,  — 
the  indestructibility  of  matter  and  the  indestructibility 
of  force. 

General  Laws  of  Chemical  Action.  —  The  following  are 
a  few  general  laws  relating  to  the  results  of  chemical 
action  :  — 

The  Law  of  Insolubility.  —  When  there  are  brought 
together  solutions  that  contain  several  elements  such  as 
would,  if  united,  form  a  compound  that  is  ordinarily 
insoluble  in  the  liquid  present,  this  insoluble  compound 
will  usually  be  formed  and  will  appear  as  a  precipitate. 

This  law  is  subject  to  certain  limitations,  yet  it  is  of 
sufficiently  wide  application  to  sometimes  enable  the 
chemist  to  predict  the  formation  of  a  given  substance 
that  may  never  have  been  produced  before  in  that  par- 
ticular way. 

This  law  finds  illustration  in  the  following  equations :  — 

HC1   +Ag(N03)=AgCl-rH(N03); 
NaCl  +  Ag(N03)  =  AgCl  +  Na(NO3) ; 
XC1   +AffY         =AffCl  +  XY. 


172  THE    ATTRACTION    OF    ATOMS. 

The  Law  of  Volatility.  —  When  there  are  brought 
together  substances  whose  reaction  can  produce  a  gas 
or  a  substance  that  is  volatile  at  the  temperature  of  the 
experiment,  such  volatile  or  gaseous  substance  generally 
will  be  formed,  and  will  be  liberated  with  effervescence. 

The  Indestructibility  of  Matter.  —  The  amounts  of 
weighable  matter  taking  part  in  a  chemical  change  are 
definite  ;  and  the  sum  of  the  weights  of  the  products  is 
always  equal  to  the  sum  of  the  weights  of  the  factors. 

The  Indestructibility  of  Energy.  — The  amounts  of  en- 
ergy involved  in  chemical  changes  are  definite.  When 
the  elements  of  a  chemical  compound  are  drawn  apart, 
a  certain  amount  of  energy  is  usually  absorbed.  When 
the  same  elements  come  together  to  form  a  compound, 
a  certain  amount  of  energy  is  evolved.  Now  the  amounts 
of  energy  in  these  two  cases  are  equal. 

It  is  true  that  the  energy  absorbed  or  evolved  in  such 
cases  may  vary  in  kind.  It  may  be  the  energy  of  heat 
or  that  of  light  or  that  of  electricity  in  some  of  its  modi- 
fications, or  it  may  be  some  combination  of  these.  But, 
in  any  event,  the  facts  sustain  the  doctrine  called  the 
conservation  of  energy,  which  involves  the  view  that  it 
is  impossible  for  us  to  create  or  to  destroy  energy,  just 
as  it  is  impossible  for  us  to  create  or  to  destroy  matter. 
All  that  we  can  do  is  to  change  the  particular  form 
which  the  energy  shall,  for  the  time  being,  assume. 

Criteria  of  Chemical  Action.  —  It  has  already  been 
stated  as  a  fundamental  principle  that  natural  phe- 
nomena arrange  themselves  in  series  in  which  the 
individual  members  differ  from  their  immediate  neigh- 
bors by  minute  and  sometimes  almost  indistinguishable 


THE    ATTRACTION    OF    ATOMS.  1/3 

details.  Thus  it  may  be  expected  that  the  drawing  of 
distinct  lines  of  division  will  often  be  impracticable. 
This  statement  is  applicable  to  those  different  kinds 
of  action  called  chemical  action  and  physical  action. 
While  a  multitude  of  operations  are  readily  recognized 
as  manifesting  distinct  evidences  of  chemical  change, 
and  at  a  distance  from  these  may  be  produced  changes 
that  are  referable  distinctly  to  cohesive  and  physical 
forces,  there  are  between  these  extremes  phenomena  in 
which  the  definite  signs  of  the  one  or  the  other  kind  of 
action  become  less  and  less  marked,  or  entirely  fade 
away. 

Thus,  on  a  given  occasion  an  observer  may  be  reason- 
ably in  doubt  whether  certain  intermingled  or  adjacent 
substances  undergo  or  do  not  undergo  what  is  properly 
described  as  chemical  change. 

It  is  therefore  desirable  to  consider  systematically  the 
evidences  upon  which  a  decision  must  be  reached.  The 
following  may  be  accepted  as  a  guiding  principle  :  Gain 
a  thorough  acquaintance  with  all  the  characteristics  that 
generally  attend  undoubted  chemical  changes  ;  then,  in 
a  doubtful  case,  observe  whether  one  or  several  single 
characteristics  are  distinctly  evident,  and  whether  one 
or  several  characteristics  can  be  recognized,  if  only  in  a 
feeble  and  rudimentary  degree. 

The  following  are  the  chief  evidences  of  well-marked  chemical  action  :  — 

(<z)  The  generation  of  certain  physical  forces,  as  heat,  light,  electricity. 
These  are  important  indications,  since  appliances  have  been  produced 
capable  of  detecting  very  minute  amounts  of  heat  and  of  electrical  dis- 
turbance. 

(/;)  The  production  of  new  molecules.  These  may  be  recognized  as 
follows :  — 

I.    They  possess  new  chemical  composition;   that  is,  they  contain  either 


174  THE  ATTRACTION  OF  ATOMS. 

new  elements  or  else  they  contain  the  original  elements  in  new  propor- 
tions by  weight  (and  in  case  of  gaseous  elementary  substances,  by  volume 
as  well).  This  evidence  involves  the  important  law  of  definite  proportions. 
(See  p.  72.) 

2.  They  possess  properties  differing  more  or  less  distinctly  from  those 
of  the  original  elementary  or  compound  molecules  which,  in  the  case  in 
question,  are  supposed  to  have  been  subject  to  chemical  change.  The 
changes  to  be  looked  for  are  in  the  following  features :  — 

The  color;   degree  of  opacity;   refracting  power  for  light; 

The  taste;   physiologic  and  toxic  properties; 

The  conducting  power  for  heat,  light,  and  electricity; 

The  density  of  the  substance  in  the  solid,  liquid,  or  gaseous  condition; 

The  melting  and  boiling  points; 

The  degree  of  solubility  in  solvents. 


CHAPTER    XVI. 

THE  ATTRACTION  OF  ATOMS  (continued}. 
THERMO-CHEMISTRY. 

Introduction.  —  It  has  long  been  recognized  that  many 
chemical  changes  give  rise  to  an  evolution  of  heat. 
Sometimes  the  amount  evolved  is  very  large.  Indeed, 
practically  all  man's  artificial  heat  is  the  product  of 
chemical  combination. 

It  is  also  well  known  that  the  amount  of  heat  evolved 
by  the  combustion  —  or  other  chemical  reaction  —  of  a 
certain  weight  of  one  substance  is  very  different  from 
that  given  out  by  a  corresponding  chemical  reaction  of 
an  equal  weight  of  another  substance. 

At  a  given  moment  of  time  any  material  substance  or 
thing  possesses  certain  internal  and  external  relations 
of  parts,  and  contents  of  heat  and  other  forces,  the  sum 
total  of  which  may  be  called  its  condition.  Now  any 
change  whatever  of  its  condition  implies  either  some 
alteration  of  the  arrangement  of  its  parts  externally  or 
internally,  or  some  alteration  by  increase  or  decrease  of 
the  amount  of  its  forces.  In  either  case  the  change 
requires  for  its  initiation  the  application  of  some  ex- 
ternal force,  which  may,  perhaps,  be  small  in  amount. 
But  the  change,  when  once  it  has  been  instituted,  either 
absorbs  or  liberates  a  large  amount  of  energy  of  some 
kind,  generally  that  of  heat. 

175 


176  THE  ATTRACTION  OF  ATOMS. 

Reactions  of  the  sort  referred  to  are  sometimes  classed  as  direct,  or 
exothermic  (those  in  which  heat  is  evolved),  and  indirect,  or  endothermic 
(those  in  which  heat  is  absorbed). 

It  is  worthy  of  note  that  the  amount  of  heat  involved  by  the  union  of 
substances  in  an  exothermic  reaction  is  exactly  equal  to  the  amount  of  heat 
that  would  be  required  to  subsequently  decompose  the  substance  produced 
by  such  operation. 

The  facts  here  stated  seem  to  be  merely  forms  of  the  general  law  of 
nature,  that  changes  in  the  arrangement  of  material  substances  cannot  be 
accomplished  without  the  aid  of  force.  In  other  words,  to  bring  about  a 
certain  change,  heat  or  other  force  must  be  supplied,  and  the  energy 
appears  to  be  somehow  taken  in  by  the  compound.  In  the  reversal  of  the 
same  operation  the  energy  taken  in  is  given  out :  then  force  is  evolved, 
as  heat  or  in  some  other  form,  equivalent  in  amount  to  that  originally 
absorbed. 

Within  a  few  years  efforts  have  been  made  to  learn  and  state  with 
exactness  the  amounts  of  heat  afforded  by  all  the  more  prominent  chemical 
tions. 

<The  exact  studies  of  Alexander  Naumann,  Julius  Thomsen,  Marcellin 
Berthelot,  and  others  rank  among  the  classical  scientific  researches  of  this 
era.  The  whole  subject  has  also  been  carefully  reviewed  in  recent  treatises 
by  Pattison  Muir  and  others. 

Laws  of  Thermo-Dynamics.  —  First  Law.  There  is 
a  definite  quantitative  relation  between  the  amount  of 
work  done  and  the  quantity  of  heat  produced  or 
destroyed. 

Second  Law. — If  all  the  heat  in  any  body  or  system 
of  bodies  is  at  the  same  temperature,  no  mechanical 
work  can  be  obtained  from  that  body  or  system  except 
by  bringing  it  into  contact  with  another  body  at  a  lower 
temperature. 

The  important  principle  stated  by  Clerk  Maxwell  may 
with  propriety  be  presented  here  :  — 

"  The  total  energy  of  any  material  system  is  a  quan- 
tity which  can  neither  be  increased  nor  diminished  by 
any  action  between  parts  of  the  system,  though  it  may 


THE    ATTRACTION    OF    ATOMS.  I// 

be  transformed  into  any  of  the  forms  of  which  energy 
is  susceptible." 

The  Law  of  Maximum  Work.  —  Berthelot  States  this 
law  as  follows  :  — 

"  Every  chemical  change  accomplished  without  the 
addition  of  energy  from  without  tends  to  the  formation 
of  that  body  or  system  of  bodies,  the  production  of  which 
is  accompanied  by  the  evolution  of  the  maximum  quan- 
tity of  heat." 

Muir  makes  a  critical  examination  of  this  statement,  and  thinks  that 
Berthelot  has  fallen  into  error.  Thus  consider  at  the  outset  the  suggestion 
that  any  chemical  change  can  be  accomplished  without  the  addition  of 
energy  from  without.  Initial  outward  influence  seems  necessary  to  change 
the  condition  of  any  portions  of  matter  not  in  actual  process  of  change. 

Thermal  Units.  —  The  amount  of  heat  absorbed  or 
evolved  in  chemical  operations  is  usually  represented  in 
thermal  units  called  calories. 

Thomsen  uses  the  water  calory,  and  his  unit  of  heat 
is  the  amount  of  heat  necessary  to  raise  one  gramme  or 
one  kilogramme  of  water  through  one  degree  measured 
in  the  neighborhood  of  the  eighteenth  to  the  twentieth 
degree  of  the  centigrade  thermometer. 

Berthelot  prefers  to  use  water  at  zero  degrees  centigrade. 

In  some  cases  an  ice  calory  is  used,  the  heat  being 
measured  by  the  amount  of  ice  that  may  be  changed 
from  the  solid  to  the  liquid  form  without  rise  of  tempera- 
ture (one  ice  calory  is  equal  to  80.025  water  calories). 

It  should  be  noted,  also,  that  sometimes  the  so-called 
large  calories  (C)  are  used,  and  sometimes  small  calo- 
ries (c).  The  large  calory  relates  to  the  kilogramme  of 
water,  the  small  calory  to  the  gramme  of  water. 


1/8 


THE    ATTRACTION    OF    ATOMS. 


The  heat  of  combustion  of  an  element  is  sometimes 
defined  as  the  amount  of  heat  evolved  by  the  perfect 
combustion  of  one  gramme  or  one  kilogramme  of  the 
substance. 

Sometimes  the  heat  of  combustion  means  the  quantity  of  heat  produced 
by  the  chemical  change  of  a  number  of  grammes  represented  in  a  certain 
reaction  of  the  substance. 

In  accordance  with  the  first  definition, 
the  heat  of  combustion  of  hydrogen  is  the 
amount  of  heat  produced  by  burning  one 
gramme  of  it.  In  accordance  with  the 
second  definition,  the  heat  of  combustion 
of  hydrogen  is  the  amount  of  heat  pro- 
duced by  burning  two  grammes  of  it 
(representing  one  molecule  of  hydrogen) 
in  accordance  with  the  equation  — 

H2  +  O  =  H2O. 


Calorimeters.  —  In  the  ex- 
periments of  thermo-chemistry 
several  different  kinds  of  ap- 
pliances have  to  be  used. 

The  forms  of  chemical  change 
are  so  varied  as  to  the  sub- 
stances taking  part,  as  to  the 
substances  ultimately  produced, 
and  as  to  the  conditions  attend- 
ing the  progress  from  the  one 
set  of  substances  to  the  other, 
that  a  thermal  study  of  these 
of  apparatus  specially  adapted 

calorimeters  are  :  First,  an  inte- 
e.g.  of  glass  or  of  platinum,  in 


FIG.  117.  —  Simple  form  of  calor- 
imeter. The  vessel  E  represents  a 
box  placed  on  a  felt  or  wooden  sup- 
port, and  provided  in  its  interior 
with  a  non-conducting  wreath  for 
thermal  insolation.  The  thermom- 
eter registers  the  heat  generated  in 
the  process  of  a  chemical  reaction. 

changes  demands  forms 
to  the  different  cases. 

The  essential  parts  of 
rior  vessel  of  some  sort, 


THE  ATTRACTION  OF  ATOMS. 


179 


which  the  chemical  operation  proceeds.      Second,  one  or 
more  delicate  and  accurate  thermometers,  to  be  used  in 


FIG.  118.  —  One  of  Berthelot's  calorimeters,  constructed  for  observing  the  heat  gen- 
erated by  mixing  chemical  substances  in  solution.  The  outer  jackets  are  for  the  purpose 
of  preventing  heat  from  passing  from  the  room  into  the  apparatus  or  from  the  apparatus 
outward.  The  beaker  in  which  the  experiment  is  performed  is  placed  upon  pointed  sup- 
ports for  thermal  insolation.  A  stirrer,  a,  is  provided  to  diffuse  throughout  the  solution 
the  heat  generated.  A  delicate  thermometer  suspended  in  the  solution  registers  the 
temperature. 


measuring  the   rise    of   temperature   of   the 
TJiird,   several   protecting  coatings   or  chan 


operation, 
chambers,   such 


i8o 


THE    ATTRACTION    OF    ATOMb. 


as  vessels  of  water  or  of  air,  covered  with  felt.  FourtJi, 
mechanical  stirrers  to  agitate  the  water  of  the  outer 
chamber  so  that  tHe  heat  absorbed  may  be  evenly  dis- 


FIG.  119.  —  Calorimeter  devised  by  Berthelot  for  experiments  upon  the  union  of  cer- 
tain gases.  The  products  of  chemical  union  are  collected  in  the  spiral  S  and  the  bulb  R. 
The  heat  generated  is  absorbed  in  the  liquid  surrounding  the  spiral.  The  thermometer 
registers  the  heat  produced.  The  vessel  containing  the  spiral  is  thermally  isolated  by 
the  several  jackets  placed  about  it. 


THE    ATTRACTION    OF    ATOMS.  l8l 

tributed.  FiftJi,  in  some  cases  the  room  in  which  the 
experiments  are  performed  is  most  carefully  maintained 
at  a  uniform  temperature.  Sixth,  sometimes  the  ex- 
periments have  a  duration  of  several  days,  so  that  the 
water  employed  as  a  protective  coating  (and  the  air 
of  the  room  also)  may  acquire  a  uniform  tempera- 
ture. 

I.  One   general   form  of  apparatus  is  suited   to  experiments  on  the 
relations  of  solids  with  water  or  other  liquids.     Under  this  head  come  such 
cases  as  the  solution  of  many  salts  in  water,  the  reaction  between  water 
solutions  of  acids  and  water  solutions  of  alkalies,  and  the  like. 

II.  Another  kind  of  apparatus  is  necessary  in  the  study  of  combustion 
processes.     Such  are  those  in  which  solids,  liquids,  or  gases  are  burned  in 
oxygen  gas,  or  are  made  to  unite  —  by  a  process  analogous  to  oxygen  com- 
bustion —  with  sulphur,  chlorine,  bromine,  or  other  substance. 

III.  Another  form  may  be  necessary  in  the  study  of  the  violent  changes 
involved  in  the  action  of  chemical  agents  on  certain  organic  matters,  as,  for 
example,  of  nitric  acid  on  sugar. 

IV.  Some  chemical  operations  may  demand  apparatus  for  their  indi- 
vidual treatment,  as,  for  instance,  when  two  gases  act  on  each  other  at 
ordinary  temperature  :   the  union  of  nitrogen  dioxide  and  oxygen  affords  an 
illustration.     Again,  the  action  of  certain  substances  on  others  proceeds 
slowly,  and  special  apparatus  may  be  needed  to  deal  with  such  changes : 
the  action  of  oxygen  on  a  solution  of  sodic  thiosulphate    (sodic  hypo- 
sulphite) affords  an  illustration. 

Difficulties  Experienced.  —  Exact  determinations  of  the 
amounts  of  heat  evolved  or  absorbed  by  chemical  opera- 
tions are  attended  with  certain  difficulties  :  — 

FIRST.  There  are  the  mechanical  difficulties  associ- 
ated with  the  construction  and  use  of  the  apparatus 
required. 

SECOND.  In  certain  distinctly  connected  series  of 
chemical  changes  heat  is  both  absorbed  and  evolved. 
The  computation  is  thereby  complicated. 


1 82  THE    ATTRACTION    OF    ATOMS. 

The  chemical  union  of  hydrogen  and  chlorine  would  be  a  simple  one  if 
it  were  properly  expressed  by  the  equation  H  +  Cl  =  HC1.  In  this  expres- 
sion a  simple  and  direct  union  of  two  atoms  is  described. 

But  the  true  change  taking  place  when  hydrogen  and  chlorine  unite  is 
believed  to  be  expressed  by  the  equation  H?  -f  C12  =  2  HC1.  This  expres- 
sion describes  something  more  complicated  than  the  foregoing.  It  ex 
presses  at  least  three  operations;  viz.  a  decomposition  of  a  molecule  of 
hydrogen,  a  decomposition  of  a  molecule  of  chlorine,  the  union  of  an  atom 
of  hydrogen  with  an  atom  of  chlorine  (this  particular  operation  being 
twice  repeated).  Now  it  may  be  safely  assumed  that  the  decomposition  of 
the  hydrogen  molecule  and  the  decomposition  of  the  chlorine  molecule 
both  absorb  heat,  while  the  union  of  the  atoms  of  hydrogen  with  the 
atoms  of  chlorine  evolves  heat.  Evidently,  then,  the  total  amount  of  heat 
observed  in  such  an  operation  represents  a  remainder  equal  to  the  excess 
of  the  evolved  over  the  absorbed  heat. 

H'=H-  (h'+h). 

H'  represents  net  observed  evolution  of  heat. 

.//represents  amount  of  heat  evolved  by  actual  chemical  union  of  the 
substances. 

h  represents  amount  of  heat  absorbed  by  decomposition  of  one  of  the 
molecules  involved. 

hf  represents  amount  of  heat  absorbed  by  decomposition  of  the  other 
molecule  involved. 

THIRD.  In  many  cases  of  chemical  change  the  heat 
actually  generated  may  be  partly  expended  in  raising 
the  temperature  of  the  solids,  liquids,  or  gases  produced. 
Of  course  any  such  rise  of  temperature  as  is  observed  in 
the  experiment  must  therefore  be  subjected  to  correction 
because  of  the  different  specific  heats  of  the  substances 
present.  Thus  the  experimenter  in  thermo-chemistry 
must  make  a  careful  study  of  specific  heat,  and  the 
results  of  thermo-chemistry  cannot  be  relied  upon  unless 
correct  results  in  specific  heat  are  employed. 

FOURTH.  In  many  cases  the  final  result  is  complicated 
because  the  substances  first  produced  dissolve  in  the 
water  present,  absorbing  or  evolving  heat  by  this  opera- 
tion. 


THE    ATTRACTION    OF    ATOMS.  183 

Range  of  the  Subject.  -  -  Evidently  the  field  of  thermo- 
chemistry is  a  wide  one. 

As  a  method  of  observation  it  has  been  brought  to 
bear  upon  many  diverse  forms  of  action.  Thus  it  has 
been  applied  to  the  study  of  the  following  classes  of 
subjects  :  — 

FIRST.  The  amounts  of  heat  of  formation  of  certain 
binary  compounds,  as  the  compounds  of  hydrogen, 
chlorine,  bromine,  iodine,  sulphur,  oxygen,  and  other 
elements  by  various  methods.  These  include  energetic 
changes  in  which  various  substances,  elementary  or 
compound,  unite  with  oxygen  or  other  elements  by  a 
process  analogous  to  combustion. 

SECOND.  The  heats  afforded  by  neutralization  of 
alkalies,  like  soda,  potash,  and  others  by  acids. 

THIRD.  The  heats  of  solution  of  solids,  the  dilution  of 
liquids,  and  hydration  generally. 

FOURTH.  The  phenomena  of  dissociation,  and  the  so- 
called  abnormal  vapor  densities. 

FIFTH.  Certain  allotropic  and  isomeric  substances. 
Thus  there  has  been  made  an  examination  of  the  differ- 
ences in  the  thermal  value  of  the  combustion  of  different 
kinds  of  sulphur,  different  kinds  of  phosphorus,  different 
kinds  of  carbon,  different  kinds  of  silicon,  as  well  as  of 
different  isomeric  compounds  of  the  organic  series,  with 
a  view  of  detecting,  if  possible,  the  differences  of  molec- 
ular structure  of  these  elements  and  compounds. 

Results.  —  While  it  is  not  possible  as  yet  to  state 
many  distinct  laws  as  to  the  heat  of  chemical  union,  it 
has  been  noticed  that  certain  operations  of  a  similar 
chemical  type  involve  approximately  equal  amounts  of 


184  THE  ATTRACTION  OF  ATOMS. 

heat,  even  when  the  particular  substances  taking  part 
are  all  different. 

The  amount  of  heat  produced  when  certain  substances  are  dissolved  in 
water  affords  important  information  as  to  the  condition  of  such  substances 
in  solution.  Thus  it  is  well  known  that  upon  mingling  sulphuric  anhydride 
(SO3)  with  water,  heat  is  produced.  A  critical  study  of  this  operation 
shows  plainly  that  a  large  proportion  of  the  heat  afforded  is  produced  by 
the  addition  of  the  first  molecule  of  water,  and  a  large  proportion  is  also 
produced  by  the  addition  of  the  second  molecule  of  water.  Muir  says  that 
there  can  be  little  or  no  doubt  that  the  various  results  point  to  the  formation 
in  aqueous  solutions  of  sulphuric  anhydride  (SO3),  of  one  definite  hydrate 
having  the  formula  H2SO4,  and  not  of  other  hydrates. 

The  same  general  results  are  obtained  by  the  study  of  the  solution  of 
other  acids. 

"  The  difference  commonly  expressed  in  the  terms  water  of  constitution 
and  water  of  crystallization  is  evidently,  so  far  as  may  be  judged  from 
thermo-chemical  data,  strictly  a  difference  of  degree  and  not  of  kind." 

A  few  examples  of  the  numerical  results  may  be  pre- 
sented here.1 

(a)    Union  of  Elements. 

H  +C1=HC1+ 22,000  cal. 
H  +  Br  =  HBr  +  8,440  " 
H  +1  -HI  -  6,040  " 
H2+O  =  H2O+68,36o  " 
H2  +  S  =H2S  +  4,740  " 
S  +  O2  =  SO2  +71,080  " 

(d)   Union  of  Compounds  with  Water. 

H2SO4  +  H2O         =  (H2O    -H2SO4)     +6,379  cal. 

KOH  -  3  H2O  +  197  H2O  =  (KOH  -  200  H2O)  +  2,75 1   " 

1  Muir,  Elements  of  Thermal  Chemistry. 


THE  ATTRACTION  OF  ATOMS.  185 

(c)    Union  of  Acids  and  Bases. 

2  HC1  +  Na2O  =  2  NaCl  +  H2O  +  27,480  cal. 


2  HI    +  Na2O  =  2NaI    +  H2O  +  27,360   " 

In  this  last  set  of  equations  the  similarity  of  the  number  of  calories 
evolved  at  once  suggests  similarity  in  the  operations  in  question.  This 
similarity  is  noticeable  in  many  other  cases  among  elements  and  com- 
pounds of  the  same  family  group. 

Indeed,  Thomsen  classifies  the  acids  in  a  general  way  into  sets  some- 
what as  follows  :  — 

FIRST.     Acids  whose  heats  of  neutralization  are  about  20,000  cal. 
Examples  are  — 

Nitrous  acid  .............  HNO2, 

Hypochlorous  acid       ..........  HC1O. 

Carbonic  acid      ............  H2CO3, 

Metaboric  acid   ............  H2B2OV 

SECOND.     Acids  whose  heats  of  neutralization  are  about  25,000  cal. 
Examples  are  — 

Chromic  acid      ............     H2CrO4, 

Succinic  acid       ............     C4H6O5. 

THIRD.     Acids  whose  heats  of  neutralization  are  about  27,000  cal. 
Examples  are  — 

Hydrochloric  acid  ...........  HC1, 

Hydrobromic  acid  ...........  HBr, 

Hydriodic  acid   ............  HI, 

Nitric  acid      .............  HNO3, 

Chloric  acid   .............  HC1O3, 

Bromic  acid    .............  HBrO3, 

lodic  acid       .............  HIO3, 

Formic  acid    .....     ........  HOCHO, 

Acetic  acid     .............  HOC2H3O. 

Most  of  the  acids  examined  by  Thomsen  belong  to  this  group. 


1 86  THE    ATTRACTION    OF    ATOMS. 

FOURTH.     Acids  whose  heats  of  neutralization  are  greater  than  27,000 
cal.  (generally  from  28,000  to  32,500  cal.). 
Examples  are  — 

Hydrofluoric  acid HF, 

Sulphurous  acid H2SO3, 

Sulphuric  acid H2SO4, 

Selenic  acid H2SeO4, 

Metaphosphoric  acid HPO3, 

Phosphorous  acid H3PO3, 

Oxalic  acid H2O2(C2O2). 


CHAPTER   XVII. 

THE  ATTRACTION  OF  ATOMS  (continued}. 
THEORIES  OF   THE    NATURE  OF  CHEMICAL    ATTRACTION. 

THE  force  —  whatever  may  be  its  nature  — that  leads 
substances  to  undergo  chemical  changes  is  often  called 
chemical  affinity.  This  force  is  capable  of  overcoming 
a  certain  amount  of  resistance  ;  again,  a  certain  amount 
of  force  is  necessary  to  undo  its  work.  It  also  bears 
definite  quantitative  relations  to  other  forces,  such  as 
heat,  light,  and  electricity  :  definite  amounts  of  chemical 
energy  are  necessary  to  the  production  of  unit  amounts 
any  one  of  them,  and  definite  amounts  of  one  of  them  are 
necessary  to  the  production  of  a  unit  of  chemical  energy. 

A  very  large  amount  of  information  has  been  secured 
as  to  its  ways  of  working,  etc.,  but  no  entirely  satisfac- 
tory explanation  of  its  nature  has  yet  been  offered. 
Views  as  to  the  nature  of  chemical  attraction  have 
changed  from  time  to  time  as  one  phase  of  thought  or 
another  has  been  dominant.  They  have  in  a  marked 
manner  reflected  the  spirit  of  the  time  as  aroused  by 
some  great  discovery. 

i.  Early  Views.  —  The  general  opinion  of  the  alche 
mists  was  that  somehow  or  other  substances  of  like  kind 
or  origin  tend  to  combine. 

This  view  is  evidently  inadequate  —  the  more,  in  that 

187 


1 88  THE    ATTRACTION    OF    ATOMS. 

it  is  now  known  that  the  most  active  chemical  unions  are 
between  substances  that  are  in  many  ways  most  unlike. 

2.  Newton's  Theory.  —  Newton's  discovery  of  the  uni- 
versal attraction  of  masses  was  naturally  and  easily  ex- 
tended to  the  minute  particles  of  matter,  and  chemical 
attraction  was  then  held   to  be  one   form  of   general 
attraction.     This  view  was  advanced  by  Newton,  and 
later  was  supported  by  Berthollet.     These  philosophers 
considered,    however,    that   the   universal   tendency  of 
bodies  towards  each  other  was  somewhat  modified  by 
the  minuteness  of  the  particles  between  which  chemical 
changes  are  capable  of  taking  place. 

3.  The  Theory  of  a  Special  Force.  —  Chemical  attrac- 
tion has  been  viewed  as  a  unique  kind  of  force.     This 
theory,  widely  accepted  during  the  past  hundred  years, 
is  that  atoms  and  molecules  are  gifted  by  the  Creator 
with  certain  specific  tendencies  to  unite,  and  that  with 
certain  fixed  degrees  of  force.     This  supposes  the  pos- 
session by  atoms  of  an  inherent  property  called  chemical 
affinity.     The  name  applies  in  a  general  way  to  an  un- 
explained energy  somehow  residing  in  the  atom. 

4.  The   Electrical  Theory.  —  Chemical  attraction  has 
been  thought  to  be  a  phase  of  electrical  energy.     Davy, 
Dumas,    Becquerel,    Ampere,    Berzelius,    Gmelin,    and 
others   have   held    some   form    of   electrical   theory   of 
chemical   action.     The  general    notion    has   been   that 
atoms  and  molecules  are  naturally  or  may  be  artificially 
charged  with  varying  amounts  and  kinds  of  electricity. 
By  reason  of  their  condition  in  this  respect  they  are 
mutually  attracted  or  repelled,  and  with  varying  degrees 


THE  ATTRACTION  OF  ATOMS. 


189 


of  force,  somewhat  as  electrified  masses  of  matter  are. 
This  theory  derives  support  from  certain  important  and 
well-defined  facts.  For  bodies  artificially  electrified 
often  manifest  thereby  stronger  chemical  attractions ; 


FIG.  120.  —  Sir  Humphry  Davy,  Bart.  Born  in  Penzance,  England,  December  17 
1778;  died  in  Geneva,  Switzerland,  May  29,  1829. 

"  Davy,  when  not  yet  thirty-two  years  old,  occupied,  in  the  opinion  of  all  those  who 
could  judge  of  such  labors,  the  first  rank  among  the  chemists  of  this  or  any  other  age." 

again,   chemical  action   yields  as  a  product   a  definite 
quantity  of  electricity. 

5.  The  Theory  of  Motion.  —  Williamson's  theory  is 
that  chemical  attraction  is  a  form  of  motion.  This  view 


FIG.  I2i.  —  Andre"-Marie  Ampere.     Born  at  Lyons,  January  20,  1775;  died  at  Marseilles 
June  10,  1836.     (The  portrait  is  from  a  statue  erected  at  Lyons.) 


THE  ATTRACTION  OF  ATOMS.  IQI 

accepts  the  modern  idea  of  constant  atomic  and  molec- 
ular movement.  It  suggests  that  in  atoms  of  all  or- 
dinary molecules  a  rapid  but  regulated  interchange  is 
going  on,  so  that  in  certain  cases  a  given  atom  may  be 
continually  moving  from  one  molecule  into  another.  A 
transfer  of  this  kind  could  not  be  easily  detected  among 
molecules  of  the  same  kind,  but  among  molecules  of 
different  kinds  it  would  effect  just  such  changes  as  are 
recognized  in  many  chemical  operations.  Thus  two  or 
more  molecules  of  hydrochloric  acid  (HC1,HC1)  might 
make  an  interchange  of  atoms  without  any  easily  appre- 
ciable alteration  of  properties  of  the  substance.  But 
when  a  molecule  of  argentic  nitrate  and  a  molecule  of 
hydrochloric  acid  are  brought  into  contact,  the  inter- 
change might  produce  two  new  molecules  possessing 
properties  easily  recognized  as  different  from  those  of 
the  original  two,  — 

Ag(NOs)    +          HC1  AgCl        +    HNO3 

Argentic  nitrate      +      Hydrochloric  acid       =      Argentic  chloride      +      Nitric  acid. 

This  theory,  moreover,  not  only  explains  such  simple 
operations  as  that  just  referred  to  ;  it  is  capable  of 
affording  an  adequate  reason  for  certain  of  the  more 
obscure  phenomena  of  chemical  change. 

Comment  on  these  Theories.  —  Chemical  attraction 
must  be  looked  upon  as  a  force  having  in  it  something 
of  general  powers  and  something  of  highly  specialized 
ones.  Thus  any  theory  of  it  ought  to  include  the  notion 
that  all  substances  tend  to  come  toward  each  other,  for 
apparently  all  chemical  substances  will  combine  —  there 
is  merely  a  difference  in  the  strength  of  this  tendency 
in  different  cases.  And  so  if  masses  of  matter  gravitate 


THE    ATTRACTION    OF    ATOMS. 

toward  each  other,  why  not  molecules  and  even  atoms  ? 
Where  shall  be  drawn  the  line  at  which  gravitative 
force  ceases  ? 

Further,  it  must  be  admitted  that  substances  are  found  to  possess 
certain  natural  qualities  —  inexplicable  ones.  What  is  this  but  a  declara- 
tion that  they  are  gifted,  at  their  original  creation,  with  specific  powers? 

Again,  the  close  connection  of  electricity  with  chemical  force  is 
indubitable. 

The  modern  notion  of  constant  movement  of  all  forms  of  matter  applies 
with  peculiar  appropriateness  to  atoms  and  molecules,  and  seems  to  be 
inseparable  from  any  idea  of  so  intimate  a  change  as  the  chemical. 

There  is  no  impropriety  in  considering  chemical  attraction  as  a  complex 
rather  than  a  simple  form  of  force.  Certainly  the  rich  variety  of  its  modes 
of  action  and  of  its  results  must  sustain  such  a  view. 

Thus  each  of  the  theories  stated  contains  truths.  The  acute  observers 
and  thinkers  who  have  held  them  could  not  have  been  entirely  misled. 
Each  theory  singly  is  merely  inadequate.  Probably  in  one  that  is  adequate 
there  must  be  combined  the  truth  included  in  each  of  those  stated  —  and 
more,  too,  as  chemistry  advances. 

What  is  wanted,  then,  is  a  compact  statement,  sufficiently  comprehen- 
sive to  embrace  in  harmonious  union  the  various  principles  known  to  be 
involved  in  chemical  change. 


CHAPTER   XVIII. 
ATOMIC  WEIGHT. 

METHOD   OF   WORK  AND    METHOD    OF   DESCRIPTION. 

Introduction.  —  Whatever  theory  an  individual  may 
hold  with  respect  to  the  existence  of  atoms,  —  in  the 
most  distinct  import  of  that  word,  —  it  cannot  be  reason- 
ably questioned  that  substances  combine  chemically  in 
accordance  with  certain  approximately  fixed  proportions, 
and  that  these  proportions  may  be  expressed  in  tolerably 
exact  numerical  form. 

As  a  matter  of  fact,  chemists  assign  to  each  elemen- 
tary (and  compound)  substance  a  certain  representative 
number.  Such  numbers  are  certainly  combining  num- 
bers. They  are  probably  much  more.  For  elements, 
they  represent  at  least  an  approach  to  atomic  weights, 
and  for  compounds,  at  least  an  approach  to  molecular 
weights.  They  are,  in  fact,  compact,  single  expres- 
sions, of  the  best  form  now  known,  embracing  at  once 
in  harmonious  union,  weight  ratios,  volume  ratios,  vapor 
densities  of  elements,  vapor  densities  of  compounds, 
specific  heats  of  elements,  specific  heats  of  compounds, 
substitution  powers  of  elements  and  compounds,  and 
even  other  relations  besides. 

"  It  is  true  it  may  be  questioned  whether  there  is  an  absolute  uniformity 
in  the  mass  of  every  ultimate  atom  of  one  and  the  same  chemical  element. 
Probably  atomic  weights  merely  represent  a  mean  value  around  which  the 

193 


194  ATOMIC    WEIGHT. 

actual  atomic  weights  of  the  atoms  vary  within  certain  narrow  limits. 
When,  therefore,  it  is  said,  e.g.  that  the  atomic  weight  of  calcium  is  40, 
the  actual  fact  may  well  be  that  whilst  the  majority  of  the  calcium  atoms 
really  have  the  atomic  weight  of  40,  some  are  represented  by  39.9  or  40.1, 
a  smaller  number  by  39.8  or  40.2,  and  so  on.  The  properties  which  we 
perceive  in  any  element  are  thus  the  mean  of  a  number  of  atoms  differing 
among  themselves  very  slightly,  but  still  not  identical. "  * 

In  case  of  numbers  accepted  as  atomic  weights,  the  rule  in  this  discus- 
sion is  as  follows :  When  the  number  involves  a  fractional  part,  this  part  is 
so  modified  that  quantities  less  than  .05  are  rejected,  while  quantities  equal 
to  or  greater  than  .05  are  counted  as  I  of  the  next  higher  denomination. 

Practical  Importance  of  Atomic  Weights.  —  The  num- 
bers adopted  as  atomic  weights  have  unquestioned 
practical  value.  They  serve  as  a  basis  for  the  calcula- 
tions of  the  chemist  in  his  analytical  processes,  and  also 
as  a  foundation  for  the  work  of  all  the  great  chemical 
manufacturing  industries  of  the  world.  Their  value 
depends  in  part  upon  the  invariability  of  the  numerical 
laws  of  combination  through  all  chemical  mutations. 

Labor    expended    in    securing    Atomic   Weights.  —  The 

numbers  adopted  have  so  great  an  importance  that 
chemists  have  devoted  their  highest  skill  and  their  most 
assiduous  labor  to  the  exact  ascertainment  of  them. 
(Examples  may  be  found  in  the  work  of  Berzelius  on 
many  elements,  that  of  Stas  on  many  elements,  es- 
pecially silver,  of  Crookes  on  thallium,  Mallet  on  alu- 
minium and  gold,  Cooke  on  antimony,  and  Rayleigh, 
Cooke,  and  others  on  oxygen.) 

The  different  elements  afford  different  kinds  of  information :  Thus  the 
atomic  weight  adopted  to-day  for  one  element  may  be  worthy  of  far  greater 
confidence  than  that  adopted  for  another. 

1  W.  Crookes. 


ATOMIC    WEIGHT. 


195 


And  again,  while  the  atomic  weight  ultimately  accepted  in  a  given  case 
ought  to  be  one  which  harmonizes  with  the  entire  body  of  chemical  and 
physical  knowledge,  it  must  be  expected  that  there  should  remain  a  few 


FIG.  122.  — Jons  Jakob  Berzelius.     Born  in  East  Gothland  (in  Sweden) ,  August  20, 1799; 
died  August  7,  1848. 

exceptional  cases  incapable  of  immediate  explanation.  It  is  acknowledged 
to  be  a  matter  of  no  slight  difficulty  to  fix  upon  the  atomic  weight  as  dis- 
tinguished from  some  simple  multiple  or  fraction  of  it :  in  some  cases, 
owing  to  insufficient  data,  it  is  at  present  impossible. 


196  ATOMIC    WEIGHT. 

In  any  event  the  work  demands  two  distinct  kinds 
of  operations :  first,  the  experimental  part,  involving 
numerous  tests  and  analyses  ;  second,  a  work  that  is 
even  higher  and  more  difficult ;  i.e.  the  reasoning  part  — 
the  drawing  of  the  proper  inferences  from  the  body  of 
experimental  facts  accumulated. 

Since  Dalton's  first  attempt  to  determine  atomic 
weights  these  constants  have  assumed,  little  by  little, 
an  increasing  interest.  At  present  the  effort  to  secure 
the  most  exact  numerical  expressions  for  them  is  con- 
sidered by  chemists  a  work  of  the  highest  importance. 

Methods  of  Determination.  —  In  determining  the  atomic 
weight  of  a  given  element,  a  connected  series  of  steps 
must  be  taken.  The  following  is  a  brief  outline  of 
them  :  — 

First  Step.  —  Adopt  a  suitable  unit  for  the  system. 

Second  Step.  —  Fix  a  basis  upon  which  shall  be 
selected  the  compounds  (of  the  element  sought}  to  be 
studied. 

Third  Step.  —  Proceed  to  make  gravimetric  analyses 
of  the  selected  compounds.  From  these  discover 
directly  a  combining  number  for  the  element. 

Fourth  Step.  —  Make  choice  of  an  atomic  weight  for 
the  element  from  the  various  multiples  or  submultiples 
of  the  combining  number  discovered.  In  doing  this,  be 
guided  by  certain  facts  combined  and  applied  in  accord- 
ance with  definite  principles. 

Thus,  learn  — 

The  vapor  density  of  the  element ; 

The  vapor  density  of  its  compounds ; 

The  volume  composition  of  the  compounds ; 


ATOMIC    WEIGHT.  1 97 

The  specific  heat  of  the  element  ; 

Any  other  suitable  data. 

Fifth  Step.  —  Confirm  tJie  foregoing  choice  as  fully  as 
possible.  In  doing  this,  employ  as  many  chemical  and 
physical  facts  as  possible.  With  this  in  view,  study  the 
clement  and  its  compounds  in  connection  with  molec- 
ular formulas. 

These  involve  a  consideration  of  — 

Molecular  grouping ; 

Specific  heats  of  compounds  ; 

The  boiling-points  of  compounds  ; 

The  crystalline  forms  ; 

Such  other  relationships  as  may  be  useful. 

Sixth  Step.  —  Bring  all  the  atomic  weights  obtained 
into  one  table  and  arrange  them  in  an  appropriate  order. 
This  has  been  attempted  by  Newlands,  Mendeleeff,1  and 
many  others.  The  two  investigators  mentioned  have 
been  among  the  most  successful. 

NOTE.  It  may  be  noted  here  that  it  is  the  gravimetric  analyses  that 
give  the  results  that  are  numerically  capable  of  the  highest  degree  of  accu- 
racy. Indeed,  they  afford  the  only  decisive  foundation.  The  numbers 
they  afford  are  certainly  combining  numbers. 

On  the  other  hand,  the  various  vapor  densities  and  the  various  specific 
heats  are  mainly  valuable  as  guiding  in  the  choice  between  the  several 
multiples  of  a  number  already  learned. 

The  Method  of  Discussion.  —  It  is  plain  that  the  subject  in  hand 
is  an  extended  one.  There  is  some  difficulty  even  in  the  selection  of  a 
method  of  presenting  it.  Several  ways  are  open. 

It  is  not  well  to  follow  here  the  exact  historical  course  of  the  subject, 
for  this  has  been  marked  by  tentative  and  even  erroneous  views.  However 
instructive  these  may  be  to  the  experienced  chemist,  they  can  only  embar- 
rass the  beginner. 


1  Transliteration  of  Russian  words  :  Nature,  xli.  396;  xlii.  6,  77,  316. 


198  ATOMIC    WEIGHT. 

It  seems  preferable  to  carry  the  student  over  a  course  directly  leading 
to  what  is  now  accepted  as  truth ;  but  the  course  to  be  selected  should 
accord  with  the  natural  progress  and  be  shaped  by  a  sound  pedagogic 
method. 

In  the  following  discussion  certain  numbers  are  very  quickly  adopted. 

This  is  not  improper.  But  it  should  not  be  forgotten  that,  as  already 
intimated,  the  historical  progress  of  chemistry  has  involved  a  much  more 
circuitous  route  to  reach  these  numbers  than  the  discussion  suggests. 


CHAPTER    XIX. 
ATOMIC  WEIGHT  (continued). 
FIRST   STEP:    A   UNIT   ADOPTED. 

THE  subject  involves  the  search  for  certain  numbers. 
Now  it  must  be  remembered  that  all  numerical  expres- 
sions of  mixed  mathematics  involve  the  use  — either  ex- 
pressed or  implied  —  of  some  unit;  and  often  the  unit 
chosen  depends  more  upon  convenience  than  upon  any 
other  consideration.  These  statements  apply  to  atomic 
weights.  Some  unit  has  to  be  selected.  At  the  present 
day  the  one  almost  universally  accepted  is  the  weight  of 
a  single  atom  of  hydrogen,  —  a  weight  extremely  small, 
but  one  that  it  is  possible  (though  not  necessary)  to 
express  in  terms  of  every-day  weights. 

It  is  believed  that  the  weight  of  a  single  atom  of  hydrogen  is  equal 
to  35  grammes  divided  by  io23,  —  an  amount  about  equal  to  one  and  one 
quarter  ounces  divided  by  one  million  million  million  million.  This  minute 
weight  has  received  the  special  name  microcrith.  When,  therefore,  it  is 
said  that  the  atomic  weight  of  oxygen  is  16,  the  meaning  is  that  a  single 
atom  of  oxygen  weighs  16  microcriths. 

A  Different  Unit  might  be  used.  —  It  must  be  remem- 
bered that  this  selection  of  the  atom  of  hydrogen  as  the 
standard  is  a  matter  of  convenience  partly,  being  based 
on  the  fact  that  no  other  atom  has  so  low  a  weight. 
The  weight  of  any  other  element  might  be  used  if  it 
were  found  to  be  more  convenient. 

199 


2OO  ATOMIC    WEIGHT. 

Dalton  used  hydrogen  in  his  first  table  of  atomic 
weights.  Subsequently  Berzelius  and  others  recom- 
mended using  oxygen  as  the  standard,  calling  its  weight 
loo.1  This  latter  system,  however,  was  found  to  have 
the  objection  of  affording  in  many  cases  numbers  too 
large  for  convenience.  Thus,  when  the  atomic  weight 
of  oxygen  equals  100,  the  atomic  weight  of  uranium 
becomes  about  1494. 

Certain  practical  objections  have  been  urged  recently  against  the  em- 
ployment of  the  atom  of  hydrogen  as  the  unit  with  the  atomic  weight,  I. 

Thus  it  is  very  difficult  to  decide  upon  the  exact  ratio  of  the  weight  of 
the  hydrogen  atom  to  that  of  the  oxygen  atom.  A  number  of  ratios  have 
been  obtained  as  a  result  of  extremely  careful  investigation.  Thus  it  has 
been  placed  as  low  as  H  :  O : :  1 :  15.869,  and  as  high  as  H  :  O : :  I  :  1 6.010. 
But  since  oxygen  is  the  starting-point  for  the  determination  of  the  atomic 
weights  of  a  great  many  other  elements,  any  error  in  the  adopted  ratio 
of  H :  O  is  transferred  to  nearly  all  the  other  atomic  weights.  Thus,  if 
O  =  15.869,  then  the  atomic  weight  of  uranium  =  237.14.  If  O  =  16.010, 
the  atomic  weight  of  uranium  =  239.25. 

To  avoid  this  difficulty  it  has  been  proposed,  while  using  hydrogen  as 
the  nominal  basis  of  the  system,  to  use  oxygen  as  the  practical  basis;  to 
call  the  atomic  weight  of  oxygen  16,  and  then  to  hereafter  determine  what 
the  exact  atomic  weight  of  hydrogen  is.  At  present  it  would  be  about 
1.0025;  but  it  might  be  expected  to  be  slightly  modified  from  time  to  time 
as  the  ratio  of  the  combining  portions  of  oxygen  and  hydrogen  is  deter- 
mined with  greater  and  greater  exactness.  Meanwhile,  changes  in  this 
ratio  would  not  necessarily  alter  the  entire  scheme  of  atomic  weights. 

This  proposition,  to  adopt  oxygen  with  atomic  weight  16  as  the  unit,  has 
received  the  approval  of  many  eminent  chemists. 

SECOND   STEP:     SELECTION   OF  THE   COMPOUNDS   AND 
THE   PROCESSES  TO   BE   EMPLOYED. 

In  a  certain  sense  every  chemical  compound  (and 
every  chemical  process)  is  capable  of  contributing  some- 

1  Meyer,  L.,  and  Seubert,  K.,  American  Chemical  Journal,  vii.  96. 


ATOMIC    WEIGHT.  2OI 

thing  to  the  knowledge  of  atomic  weights  ;  and,  in  fact, 
the  study  of  a  large  number  of  them  is  necessary.  But 
certain  ones  are  far  more  serviceable  than  others. 

The  compounds  must  be  selected,  then,  with  definite 
principles  in  mind,  and  they  should  conform  to  as  many 
as  possible  of  the  following  requisites  :  — 

1.  They  should  be  such  as  can  be  prepared  in  a  form 
possessing  a  high  degree  of  purity. 

Moreover,  in  purifying  the  materials  used  for  analysis 
the  so-called  "fractional"  methods  should  be  employed. 

2.  As  many  different  compounds  as  practicable  should 
be  tested.     This  helps  the  analyst  to  avoid  "constant" 
sources  of  error. 

3.  They  should  be  such  as  are  capable  of  having  their 
composition  determined  with  a  high  degree  of  exactness. 

With  this  in  view  the  analyses  should  require  as  simple 
processes  as  possible. 

"  Improvements  made  of  late  in  manipulative  methods  and  apparatus 
have  tended  to  reduce  very  much  the  magnitude  of  what  are  commonly 
called  '  fortuitous '  errors  in  quantitative  determinations  of  matter,  and  to 
increase  greatly  the  accuracy.  No  one  nowadays  would  undertake  the 
determination  of  an  atomic  weight  of  one  of  the  better-known  elements 
without  taking  such  elaborate  precautions  as  must  practically  insure  pretty 
close  concordance  of  results  when  obtained  by  the  same  method  applied 
in  the  same  hands.  But  such  mere  close  agreement  is  not  alone  sufficient." 

4.  They  should  be  such  as  possess  a  molecular  con- 
dition that  is  simple  and  can  be  distinctly  ascertained. 

Thus  it  is  well  to  employ  compounds  of  only  two 
elements.  Again,  if  a  given  pair  of  elements  forms  a 
series  of  compounds,  that  one  containing  the  smallest 
amount  of  the  element  under  consideration  is  most 
likely  to  contain  one  atom  of  it. 

Again,  compounds  that  are  gaseous  at  ordinary  tern- 


2O2  ATOMIC    WEIGHT. 

peratures  or  that  can  be  easily  vaporized,  are  employed. 
(See  p.  37.) 

5.  In  reactions  depended  upon,  only  such  other 
elements  should  be  concerned  as  may  be  counted 
among  those  of  which  the  atomic  weights  are  already 
known  with  the  nearest  approach  to  exactness. 

Thus,  (#)  Compounds  with  hydrogen  are  preferred  when  they  are 
practicable ;  for  hydrogen  has  generally  been  adopted  as  the  basis  of  the 
system,  and  so  involves  little  or  no  error. 

Indeed,  there  are  four  compounds  of  hydrogen  so  well  suited  to  this 
purpose  that  they  have  been  called  the  type  compounds  of  modern  chemis- 
try; they  are  — 

Hydrochloric  acid  (hydrogen  and  chlorine) ; 

Water  (hydrogen  and  oxygen)  ; 

Ammonia  gas  (hydrogen  and  nitrogen) ; 

Marsh  gas  (hydrogen  and  carbon). 

(^)  In  case  hydrogen  compounds  are  impracticable,  then  the  com- 
pounds selected  should,  if  possible,  be  such  as  contain  certain  atoms  that 
have  been  compared  directly  and  quantitatively  with  hydrogen. 

The  majority  of  the  elements  do  not  form  hydrogen  compounds,  so 
that,  in  fact,  recourse  is  oftener  had  to  oxygen  compounds  and  chlorine 
compounds.  All  the  elements  (except  fluorine,  and  that  combines 
with  hydrogen)  form  oxides,  and  oxygen  itself  has  been  compared  with 
hydrogen  with  a  high  degree  of  accuracy.  Again,  chlorine  combines 
with  a  great  many  metals,  and  the  atomic  weight  of  chlorine  has  been 
accurately  determined. 

If  oxygen  is  adopted  (with  the  number  1 6)  as  the  basis  of  the  system, 
then  compounds  of  oxygen  will  be  selected,  even  in  preference  to  com- 
pounds of  hydrogen. 

6.  Further,  it  is  desirable  that  as  few  other  elements 
as  possible  —  the  assumed  atomic  weights  of  which  will 
have  to  be  taken  into  account  —  shall   be  involved  in 
each  single  reaction  depended  upon. 

7.  In  selecting  different  processes  to  be  applied    to 
the   determination    of    the    atomic    weight    of   a    given 
element,  it  is  desirable  that  not  the  same,  but  as  many 


ATOMIC    WEIGHT.  2O3 

different  other  elements  as  possible,  shall  be  concerned 
in  the  several  reactions. 

Study  of  the  Reactions.  —  Careful  preliminary  study  is  required 
as  to  the  general  effect  of  each  reaction  involved,  and  as  to  how  it 
may  be  influenced  by  the  conditions  of  the  experiment.  For  it  has  been 
learned  more  and  more  of  late  that  many  reactions — perhaps  it  should 
rather  be  said  all  reactions  —  which  have  been  generally  supposed  to  be  of 
the  simplest  nature,  are,  in  reality,  complex. 

As  many  different  and  independent  processes  as  can  be  devised 
(reasonably  free  from  apparent  sources  of  error)  should  be  employed. 

Each  process  employed  should  be  as  simple  as  possible,  both  in  the 
kind  of  chemical  changes  involved  as  well  as  in  liability  to  manipulative 


Comparison  of  Results.  —  In  comparison  of  results,  careful  con- 
sideration should  be  given  as  to  the  probable  influence  of  each  kind  of 
experiment;  i.e.  whether  it  tends  on  the  whole  to  yield  higher  results  or 
lower  results  than  the  truth.1 

1  Mallet,  J.  W.,  American  Chemical  Journal,  xii.  82. 


CHAPTER    XX. 
ATOMIC  WEIGHT  (continued). 

THIRD  STEP:    THE  EXPERIMENTAL  WORK   FOR   SECURING 
A   FEW  ATOMIC  WEIGHTS. 

THIS  stage  is  a  very  extended  one.  It  involves,  in 
fact,  all  the  experimental  work  done  and  recorded  —  up 
to  the  time  of  forming  conclusions.  Now  chemical 
work  has  been  done  —  varying  in  extent  and  accuracy 
—  upon  nearly  if  not  all  substances  known  to  civilized 
nations.  But  of  course,  for  the  purpose  of  this  discus- 
sion, only  a  few  of  the  results  can  be  referred  to. 

I. 

A  STUDY  OF  CHLORINE  AND  ITS  AFFILIATED  ELEMENTS, 
BROMINE  AND  IODINE  (ALSO  SODIUM,  POTASSIUM,  AND 
SILVER). 

(a)    Gravimetric  Composition  of   Certain  Compounds.  — 

Experimental  Facts.  A  study  of  certain  compounds  of 
chlorine,  bromine,  and  iodine  has  afforded  a  series  of 
facts  which  are  stated  in  the  following  table :  — 


204 


ATOMIC    WEIGHT. 


205 


FIRST  TABLE. 

APPROXIMATE  PERCENTAGE  COMPOSITION  OF  TWELVE  COMPOUNDS. — 
EXPERIMENTAL  RESULTS. 

Chloride  of  Hydrogen.      Bromide  of  Hydrogen.         Iodide  of  Hydrogen. 
Hydrogen,     2.75  p.  ct.      Hydrogen,      1.24  p.  ct.      Hydrogen,       .79  p.  ct. 
Chlorine,      97.25     "          Bromine,      98.76     "          Iodine,         99.21     " 


100.00 


100.00 


Chloride  of  Sodium.          Bromide  of  Sodium.  Iodide  of  Sodium. 

Sodium,        39.38  p.  ct.      Sodium,        22.37  P-  ct-      Sodium,         15.37  p.  ct 
Chlorine,      60.62     "          Bromine,      77.63     "          Iodine,          84.63     " 


100.00 


100.00 


Chloride  of  Potassium.      Bromide  of  Potassium.        Iodide  of  Potassium. 
Potassium,    52.42  p.  ct.      Potassium,    32.83  p.  ct.      Potassium,    23.55  P-  ct* 
Chlorine,      47.58     "          Bromine,      67.17     "          Iodine,          76.45     " 


100.00 


100.00 


Chloride  of  Silver.  Bromide  of  Silver.  Iodide  of  Silver. 

Silver,  75-26  p.  ct.      Silver,  57-44  P-  ct.      Silver,  45-97  P-  ct. 

Chlorine,      24.74     "          Bromine,      42.56     "          Iodine,          54-03     " 


100.00 


100.00 


100.00 


Experimental  Results  of  the  First  Table  stated  differently. 

—  A  consideration  of  the  direct  results  given  in  Table  I  leads  to  the  detec- 
tion of  the  following  facts :  — 

1.  The  numbers  fall  into  series. 

2.  In   each    of    the   series   of    chlorides,  bromides,  and   iodides,  the 
chlorine  is   smaller   in  amount   than   the   bromine,  and   the   bromine  is 
smaller  in  amount  than  the  iodine. 

3.  When  the  amounts  of  hydrogen,  sodium,  potassium,  and  silver  are 
compared,  it  is  seen  that  their  quantities  are  in  the  order  stated;   hydrogen 
being  in  smallest  amount,  and  silver  in  largest. 

4.  If  the  three  hydrogen  compounds  are  compared  on  the  basis  of  one 
part  of  hydrogen,  the  hydrogen  series  of  compounds  shows  the  following 
composition :  — 


2O6  ATOMIC    WEIGHT. 

SECOND  TABLE. 
HYDROGEN  COMPOUNDS.  —  EXPERIMENTAL  RESULTS. 


Hydrochloric  Acid. 
Hydrogen,      I. 
Chlorine,       35.4 

364 

Hydrobromic  Acid. 
Hydrogen,      I. 
Bromine,      79.8 

80^8 

Hydriodic  Acid. 
Hydrogen,      I  . 
Iodine,        126.6 

127.6 

5.  If  trial  is  made  with  the  numbers  obtained  in  Table  2  (viz.  35.4  for 
chlorine,  79.8  for  bromine,  and  126.6  for  iodine)  in  the  other  compounds 
under  consideration,  the  following  very  remarkable  results  are  obtained :  — 


THIRD    TABLE. 

SODIUM  COMPOUNDS.  —  EXPERIMENTAL  RESULTS. 

Sodic  Chloride.                  Sodic  Bromide-  Sodic  Iodide. 

Sodium,           23.                Sodium,           23.  Sodium,            23. 

Chlorine,         35.4             Bromine,          79.8  Iodine,            126.6 

58.4                                   102.8  149.6 

FOURTH   TABLE. 
POTASSIUM  COMPOUNDS.  —  EXPERIMENTAL  RESULTS. 

Potassic  Chloride.             Potassic  Bromide.  Potassic  Iodide. 

Potassium,       39.                Potassium,       39.  Potassium,       39. 

Chlorine,         35.4             Bromine,         79.8  Iodine,           126.6 

74.4                                   118.8  165.6 

FIFTH   TABLE. 

SILVER  COMPOUNDS.  —  EXPERIMENTAL  RESULTS. 

Argentic  Chloride.           Argentic  Bromide.  Argentic  Iodide. 

Silver,            107.7             Silver,            107.7  Silver,          '  107.7 

Chlorine,         35.4             Bromine,         79.8  Iodine,           126.6 


I43-1  J87-5  234-3 


ATOMIC    WEIGHT. 


207 


The  results   are   brought    together  in    the   following 
table  :  - 

SIXTH   TABLE. 

EXPERIMENTAL  RESULTS. 


CHLORIDES. 

BROMIDES. 

IODIDES. 

Hydric  Chloride. 

Hydric  Bromide. 

Hydric  Iodide. 

Per  cent. 
Hydrogen,           2.75 
Chlorine,            97-25 

Ratio, 
i. 

35-4 

Per  cent. 
Hydrogen,         1.24 
Bromine,          98.76 

Ratio, 
i. 

79-8 

Per  cent. 
Hydrogen,             .79 
Iodine,                99.21 

100.00 

Ratio, 
i. 

126.6 

127.6 

100.00 

36-4 

100.00 

80.8 

Sod  ic  Chloride. 

Sodic  Bromide. 

Sodic  Iodide. 

Per  cent. 

Sodium,              39-38 
Chlorine,            60.62 

100.00 

Ratio. 

23- 
35-4 

Per  cent. 

Sodium,            22.37 
Bromine,          77-63 

100.00 

Ratio. 

23- 
79-8 

Per  cent. 
Sodium,               I5-37 
Iodine,                84.63 

Ratio. 

23- 
126.6 

58-4 

102.8 

100.00 

149.6 

Potassic  Chloride. 

Potassic  Bromide. 

Potassic  Iodide. 

Per  cent. 
Potassium,         52.42 
Chlorine,            47-58 

Ratio. 

39- 
35-4 

Per  cent. 
Potassium,       32.83 
Bromine,          67.17 

100.00 

Ratio. 

39- 
79-8 

118.8 

Per  cent. 
Potassium,         23.55 
Iodine,                76.45 

100.00 

Ratio. 

39- 
126.6 

165.6 

100.00 

74-4 

A  rgentic  Chloride. 

Argentic  Bromide. 

A  rgentic  Iodide. 

Per  cent. 
Silver,                 75-26 
Chlorine,            24.74 

Ratio. 

107.7 
35-4 

Per  cent. 
Silver,               57.44 
Bromine,          42.56 

Ratio. 

107.7 
79-8 

Per  cent. 
Silver,                 45.97 
Iodine,                54-°3 

100.00 

Ratio.' 

107.7 
126.6 

234-3 

100.00 

I43-I 

100.00 

187.5 

2O8  ATOMIC    WEIGHT. 

Inference  \, —  Evidently,  then,  the  following  numbers  have  some  im- 
portant fundamental  meaning :  — 

NUMBERS  WORTHY  OF  CONSIDERATION. 
Hydrogen,  H,  adopted  as i. 

['Chlorine,  Cl,  found  to  be, 35.4 

\  Bromine,  Br,      "      "    " 79.8 

I  Iodine,  I,  "      "    " 126.6 

( Sodium,  Na,       "      "    " 23.  * 

<j  Potassium,  K,     "      "    " 39. 

v.  Silver,  Ag,  "      "    " IO7-7 

(It  may  be  noted  here,  as  a  fact,  that  subsequent  study  and  comparison 
of  all  results  accessible  confirm  the  opinion  that  these  numbers  are  impor- 
tant, and  are  probably  atomic  weights.) 

Inference  2.  —  These  results  give  the  following  as  molecular  weights :  — 

Hydrochloric  acid 36.4 

Sodic  chloride 58.4 

Potassic  chloride 74.4 

Argentic  chloride I43-1 

Hydrobromic  acid 80.8 

Sodic  bromide 102.8 

Potassic  bromide       118.8 

Argentic  bromide l%7-5 

Hydriodic  acid 127.6 

Sodic  iodide 149.6 

Potassic  iodide I&5-6 

Argentic  iodide 234-3 

Inference  3.  — These  results  suggest  the  following  formulas  :  — 

H    Cl  H    Br  HI 

Na  Cl  Na  Br  Na  I 

K    Cl  K    Br  K    I 

Ag  Cl  Ag  Br  Ag  I 


CHAPTER    XXI. 
ATOMIC  WEIGHT   (continued}. 

FOURTH    STEP:     THE  CHOICE   OF  A    PARTICULAR    ATOMIC 
WEIGHT  FROM  SEVERAL  COMBINING  NUMBERS. 

(I)}  The  Density  of  Certain  Elementary  Gases  and 
Vapors.  —  Experimental  Fact  I.  When  chlorine  gas  is 
weighed  it  is  found  to  weigh,  volume  for  volume,  about 
35.4  times  as  much  as  hydrogen.  Hydrogen  is  usually 
taken  as  a  standard  of  comparison  for  weight  of  gases. 

The  number  35.4  is  called  the  density  of  chlorine  gas. 

Experimental  Fact  2.  —  A  similar  experiment  is  tried 
with  bromine  vapor.  Its  vapor  density  is  found  to  be 
about  79.9. 

Experimental  Fact  3.  — A  similar  experiment  is  tried 
with  iodine  vapor.  Its  vapor  density  is  found  to  be 
about  127. 

Inference  I.  —  Certain  numbers  are  obtained  that  are  worthy  of  atten- 
tion. Evidently  their  similarity  to  the  numbers  already  obtained  are  not 
mere  coincidences. 

Inference  2.  —  The  density  of  an  elementary  gaseous  substance  at  once 
gives  its  atomic  weight. 

Inference  3.  —  The  gaseous  state  is  a  very  favorable  one  for  study  in 
this  connection :  in  this  state  bodies  appear  to  be  in  a  sort  of  equality  of 
condition  that  favors  their  comparison  with  each  other  from  the  point,  per- 
haps, of  even  other  relations  than  atomic  weight  merely. 

(c)  The  Volume  Composition.  —  Experimental  Fact. 
When  hydrochloric  acid  gas  is  tested,  it  is  found  that 

209 


210 


ATOMIC    WEIGHT. 


two  volumes  of  the  gas  yield  by  decomposition  one  vol- 
ume of  hydrogen  gas  and  one  volume  of  chlorine  gas. 

Inference  I.  — This  sustains  the  view  assumed  in  the  preceding  study, 
that  the  compound  called  hydrochloric  acid  consists  of  one  atom  of  hydro- 
gen and  one  atom  of  chlorine.  (Of  course  it  is  possible  that  the  volume 


FIG.  123.  —  Regnault's  apparatus  for  filling  a  globe  with  gas  at  the  temperature  of  o°  C. 
previous  to  weighing,  for  the  purpose  of  determining  a  vapor  density. 


composition  merely  teaches  that  the  number  of  atoms  of  hydrogen  and  of 
chlorine  are  equal;  that  the  formula  is  HC1  or  H2C12  or  H3C13  or  HMCln. 
Until  further  information  is  secured,  chemists  assume  the  truth  of  the  sim- 
plest expression.  Further  information  is  not  in  fact  wanting,  for  subse- 
quent study  strongly  sustains  the  view  that  the  formula  is  indeed  HC1,  and 
no  other.  See  p.  228.) 

Experimental  Facts. —  Similar  results  for  hydrobromic  acid  and  hydrio- 


ATOMIC    WEIGHT. 


211 


die  acid  serve  to  confirm  the  figures  already  accepted  for  bromine  and 
iodine. 

Inference  2. — These  facts  sustain  all  the  inferences  previously  reached. 


FIG.  124.  —  Henri  Victor  Regnault.     Distinguished  French  physicist.     Born  at 
Aix-la-Chapelle  in  1810;  died  in  1878. 

(d)    The  Vapor  Density  of  Certain  Compound  Substances. 

—  Experimental  Facts.  The  vapor  density  of  hydro- 
chloric acid  gas  is  found  experimentally  to  be  about  18. 
This  means  that  a  given  volume  of  hydrochloric  acid 
gas  weighs  18  times  as  much  as  the  same  volume  of 
hydrogen  gas. 


212 


ATOMIC    WEIGHT. 


Evidently  there  exists  a  very  simple  relation  between 
the  number  representing  the  density  of  hydrochloric 
acid  gas  and  the  number  already  adopted  as  its  molec- 
ular weight ;  i.e. :  — 

18  :  36  : :  i  :          2 

Density  of  Molecular  weight  of 

hydrochloric  acid  gas.  hydrochloric  acid  gas. 

Inference  \ .  —  Perhaps  in  case  of  other 
compound  substances  the  vapor  density  and 
molecular  weight  are  connected  by  the  same 
simple  relation.  Perhaps,  as  a  rule,  the 
molecular  weight  (which  is,  from  its  nature, 
difficult  to  determine)  may  be  obtained  by 
multiplying  by  2  the  vapor  density  (which 
in  many  cases  it  is  easy  to  determine).  (By 
subsequent  experiment  this  principle  appears 
to  be  sustained,  and  the  inference  is  accepted 
as  a  just  one.) 

Inference  2.  —  Perhaps,  in  case  of  elemen- 
tary substances,  the  vapor  density  and  molec- 
ular weight  are  in  the  ratio  above  suggested ; 
that  is  I  :  2.  (By  subsequent  experiment  this 
principle  appears  to  be  well  grounded,  and 
the  inference  is  accepted  as  a  just  one.)  A 
remarkable  result  follows. 

The  vapor  density  of  chlorine  gas  is  found 
experimentally  to  be  35.4;  then  the  molec- 
ular weight  is  35.4  X  2  =  70.8.  If,  then, 
the  molecular  weight  is  70.8  (and  the  atomic 
weight  is  35.4),  then  the  number  of  atoms 
in  the  molecules  is  2,  and  the  molecular  formula  of  chlorine  is  CL2. 

(This  generalization  is  a  very  important  one.  By  subsequent  experi- 
ment it  appears  to  be  sustained  with  respect  to  most  of  the  elements  capa- 
ble of  existing  in  the  form  of  gas  or  vapor.  See  p.  249.) 

(e)  The  Specific  Heats  of  Elements.  —  Definition.  The 
specific  heat  of  a  substance  is  the  amount  of  heat  neces- 
sary to  raise  one  unit  of  weight  of  the  substance  one 


FIG.  125.  —  Method  of  intro- 
ducing into  the  bulbyl  a  portion 
of  liquid,  C,  whose  vapor  density 
is  to  be  subsequently  determined 
by  Dumas'  method. 


ATOMIC    WEIGHT.  213 

degree  of  temperature.  Water  has  a  high  specific  heat, 
and  as  it  is  usually  taken  as  the  standard,  the  specific 
heats  of  most  other  substances  are  expressed  by  decimal 
fractions.  (See  p.  46.) 

Experimental  Facts.  —  Dulong  and  Petit  made  a  great 
many  experimental  determinations  of  the  specific  heats 
of  solid  substances.  (It  is  more  difficult  with  liquids 
and  gases.)  In  case  of  elementary  solids,  whose  atomic 


FIG.  126.  —  Hot  bath,  provided  with  thermometers  for  determination  of  vapor  densi- 
ties by  Dumas'  method.  A  small  but  weighed  portion  of  liquid  is  placed  in  the  globe.  It 
is  then  vaporized  by  the  heat  of  the  bath.  Subsequently  the  tip  of  the  flask  is  sealed, 
whereupon  the  weight  of  vapor  present  at  a  certain  observed  temperature  may  be  deter- 
mined by  the  balance. 

weights  had  been  previously  determined  by  other  methods, 
they  found  that  apparently  the  higher  the  atomic  weight, 
the  lower  the  specific  heat. 

Inference.  —  They  then  enunciated  the  following  law,  now  called  the  law 
of  Dulong  and  Petit :  — 

The  specific  heats  of  solid  elements  are  inversely  proportional  to  their 
atomic  weights. 


2I4 


ATOMIC    WEIGHT. 


The  law  is  likewise  expressed  in  the  following  proportion :  — 

Specific  heat  of  A  :   Specific  heat  of  B    :  :   Atomic  weight  of  B   :   Atomic 
weight  of  A. 

This  proportion  also  discloses  the  following  fact :  The  product  of  the 
specific  heat  of  any  solid  element  by  its  atomic  weight  is  a  constant 
number. 

This  constant  is  found  to  be  about  6.3. 

Again,  the  constant  6.3  divided  by  the 
specific  heat  of  a  solid  element  yields  as  a 
quotient  the  atomic  weight  of  the  element. 


t 


FIG.  127.  —  Bunsen's  ice  calo- 
rimeter for  determining  specific 
heats  of  substances. 


FIG.  128.  —  Cooling  apparatus  for  Bunsen's  calo- 
rimeter, already  referred  to,  Fig.  127. 


Bunsen's  method  of  determining  specific  heats  of  substances  may  be 
explained  by  reference  to  Figs.  127  and  128.  5  is  a  glass  tube  carefully 
graduated  or  calibrated.  D  is  an  iron  reservoir  containing  mercury;  the 
latter  extends  from  the  line  ft  through  the  tube  C  up  into  the  tube  6".  The 
space  B  is  filled  with  water.  A  is  at  first  empty.  In  using  the  apparatus, 
intensely  cold  alcohol  is  passed  in  a  current  through  the  tube  A  by  use  of 
the  apparatus,  Fig.  128.  In  due  time  the  water  in  B  is  frozen  completely. 
The  cold  alcohol  is  then  withdrawn.  Next  the  entire  apparatus  is  placed  in 
melting  snow  or  ice  to  bring  the  whole  to  zero  centigrade.  Next  the  posi- 
tion of  the  summit  of  the  mercury  column  in  the  tube  Sis  carefully  observed. 
Now  one  gramme  of  water  at  100°  C.  is  placed  in  the  tube  A.  The  water 


ATOMIC  WEIGHT.  21$ 

melts  a  certain  portion  of  the  ice  in  B.  Thereupon  contraction  takes 
place,  as  is  indicated  by  the  rise  of  mercury  in  B  and  the  fall  of  mercury 
in  S.  The  point  to  which  the  summit  of  the  mercury  column  in  S  now 
retracts  is  observed.  The  distance  this  summit  has  traversed  corresponds 
to  the  specific  heat  of  water.  Subsequently  the  substance  to  be  tested  is 
raised  to  the  temperature  of  100°  C.  Then  one  gramme  of  it  is  placed  in 
the  tube  A.  Thereupon  it  melts  another  portion  of  ice.  Further  retrac- 
tion of  mercury  takes  place.  The  amount  of  such  retraction  being  meas- 
ured and  compared  with  the  retraction  due  to  one  gramme  of  water,  gives 
directly  the  specific  heat  of  the  substance  in  question. 

Specific  Heats  of  Compounds.  —  The  specific  heats  of  many  solid 
compounds  have  been  determined.  In  a  few  cases  they  are  in  general 
harmony  with  the  law  of  Dulong  and  Petit;  in  many  cases  they  are  not. 
In  some  cases  the  specific  heat  of  a  solid  compound  containing  two  atoms, 
and  having  a  given  molecular  weight,  appears  to  be  twice  as  great  as  that 
of  a  single  elementary  substance  of  the  same  atomic  weight;  and  the 
specific  heat  of  a  solid  compound  of  three  atoms  appears  to  be  three 
times  as  great  as  that  of  a  mere  element  whose  atomic  weight  equals 
the  molecular  weight  of  the  compound.  This  seems  to  show  that  when 
a  compound  body  is  heated,  the  rise  of  temperature  is  associated  with  a 
motion  of  each  different  atom  in  the  molecule,  and  not  merely  with  that  of 
the  molecule  as  a  whole. 

While,  then,  it  requires  a  certain  amount  of  heat  to  impart  to  an 
elementary  substance  an  amount  of  motion  sufficient  to  produce  a  certain 
change  of  temperature,  in  case  of  a  compound  body  with  the  same  molec- 
ular weight  more  heat  is  required  to  impart  to  it  the  amount  of  motion 
that  affords  the  same  temperature  as  that  already  supposed,  —  twice  as 
much  heat  is  required  for  compounds  of  two  atoms,  and  three  times  as 
much  heat  for  compounds  of  three  atoms. 

Atomic  Heats  of  Elements.  —  The  constant  6.3  is  often 
called  the  atomic  heat  of  an  element.  The  significance 
of  this  expression  may  be  explained  as  follows  :  Upon 
taking  one  unit  of  weight  of  each  of  several  elementary 
substances,  and  then  applying  equal  amounts  of  heat  to 
each  of  them,  it  is  observed  that  the  temperature  rises 
to  different  degrees  in  the  different  cases. 


2l6  ATOMIC    WEIGHT. 

Suppose,  however,  different  weights  of  the  substances  are  experimented 
upon,  say  — 

7  parts  of  lithium, 
56  parts  of  iron, 
194  parts  of  platinum, 
204  parts  of  lead; 

it  is  then  found  that  equal  amounts  of  heat  added  to  these  several  amounts 
by  weight  produce  in  all  the  same  advances  of  temperature.  Evidently, 
then,  equal  amounts  of  heat  applied  to  single  atoms  of  these  substances  will 
produce  the  same  advances  of  temperature.  This  explains  the  statement 
that  the  atomft  heats  of  the  elements  are  the  same. 

Special  Cases.  —  Indirect  Determination  of  Specific 
Heats.  In  cases  of  certain  elements  the  specific  heat 
cannot  readily  be  determined  directly.  This  is  especially 
true  of  the  gaseous  elements,  as  hydrogen,  fluorine, 
chlorine,  oxygen,  nitrogen.  But  indirect  methods  have 
been  devised.  (In  case  of  certain  solid  elements,  as  car- 
bon, boron,  and  silicon,  the  specific  heats  are  abnormal. 
This  is  supposed  to  be  due  to  the  tendency  of  these 
elements  to  assume  allotropic  modifications.) 

Indirect  Method  for  Chlorine.  —  The  specific  heat  of  argentic  chloride 
has  been  learned  experimentally.  It  is  .089.  Now  by  independent 
methods  the  molecular  weight  of  the  compound  is  found  to  be  143.1,  and 
it  is  found  to  consist  of  two  atoms,  —  one  of  silver  and  one  of  chlorine. 
Multiplying  the  molecular  weight  by  the  specific  heat  (143.1  X  .089),  the 
molecular  heat  12.7  is  obtained.  Subtracting  from  this  number  the  atomic 
heat  of  silver,  6.1  (as  experimentally  obtained),  there  remains  6.6  as  the 
atomic  heat  of  chlorine  indirectly  determined. 

Many  other  similar  indirect  determinations  for  chlorine  have  been 
made;  generally  speaking,  they  yield  the  number  6.4. 

Indirect  Method  for  Carbon. — The  specific  heat  of  carbon  hexachloride 
(C2C16)  has  been  learned  experimentally.  It  is  .177.  By  independent 
methods,  its  molecular  weight  is  found  to  be  236.4,  and  it  is  found  to  con- 
sist of  eight  atoms  as  stated.  Multiplying  the  molecular  weight  by  the 
specific  heat  (236.4  X  .177),  the  molecular  heat,  41.8,  is  obtained.  Sub- 
tracting from  this  number  six  times  the  atomic  heat  of  chlorine  (6  X  6.4  = 


ATOMIC    WEIGHT. 

38.4)  (41.8  —  38.4  =  3.4),  there  remains  3.4,  which  is  twice  the  atomic 
heat  of  carbon.  By  this  means  the  atomic  heat  of  carbon  is  1.7  indirectly 
determined. 

Other  similar  indirect  determinations  have  given  the  atomic  heat  of 
carbon,  in  combination,  as  about  2. 

Specific  Heat  of  Bromine.  —  Experimental  Fact.  The 
specific  heat  of  solid  bromine  has  been  found  by  direct 
experiment ;  it  is  .08432.  Applying  the  law  of  Dulong 
and  Petit,  i.e.  dividing  .08432  into  the  constant  6.3,  and 
there  results  the  quotient  75. 

Inference.  —  The  atomic  weight  of  bromine  is  nearly  75. 

But  the  studies  of  bromine  already  referred  to  (pp.  204  and  209)  show 
that  the  combining  number  is  about  79.8.  Its  atomic  weight  is  probably 
either  79.8,  or  2  X  79.8,  or  3  X  79-8,  or  n  X  79-8.  Now,  as  has  been  said 
before,  while  the  specific  heat  does  not  give  the  exact  atomic  weight,  it 
enables  us  to  decide  that  79.8,  and  not  some  multiple  (or  fraction)  of  it, 
should  be  accepted. 

Specific  Heat  of  Iodine.  —  Experimental  Fact.  The 
specific  heat  of  solid  iodine  has  been  found  by  direct 
experiment  to  be  .0541.  Applying  the  law  of  Dulong 

63 

and  Petit,  — — -  =  about  116. 
.0541 

Inference.  — The  atomic  weight  of  iodine  is  about  116. 
But  the  results  previously  and  otherwise  obtained  point  to  126.6.     Evi- 
dently the  specific  heat  confirms  this  selection. 

Specific  Heat  of  Sodium.  —  Experimental  Fact.  The 
specific  heat  of  sodium  is  found  directly  to  be  .293. 

Now  — -  =  about  22. 

Inference.  —  The  atomic  weight  of  sodium  is  about  22. 
But  results  previously  and  otherwise  obtained  have  given  the  number 
23.     Evidently  the  specific  heat  confirms  this  selection. 


2l8  ATOMIC    WEIGHT. 

Specific  Heat  of  Potassium.  —  Experimental  Fact.  The 
specific  heat  of  potassium  is  found  directly  to  be  .166. 

Now  — ^-  =  about  38. 
.100 

Inference.  —  The  atomic  weight  of  potassium  is  about  38. 
But  results  previously  and  otherwise  obtained  have  given  the  number 
39.     Evidently  the  specific  heat  confirms  this  selection. 

Specific  Heat  of  Silver.  —  Experimental  Fact.  The 
specific  heat  of  silver  is  found  directly  to  be  .057. 

6.3 

Now  — —  = about  in. 
•U57 

Inference.  —  The  atomic  weight  of  silver  is  about  in. 
But  results  previously  and  otherwise  obtained  have  given  the  number 
107.7.     Evidently  the  specific  heat  confirms  this  selection. 

General  Inference.  —  If  now  reference  is  made  to  the 
provisional  table  presented  on  p.  208,  it  is  seen  that  the 
numbers  there  given  secure  marked  confirmation  from 
the  experiments  subsequently  detailed. 

They  are  repeated  here  as  — 

SEVENTH   TABLE. 

WELL-ESTABLISHED  ATOMIC  WEIGHTS. 

Atomic  weight  of  hydrogen  adopted  as   ....       i. 

"  "      "  chlorine  found  to  be    .     .     .     .     35.4 

"      "  bromine  "  ....     79.8 

"       "  iodine  "  ....   126.6 

"  "      "  sodium  "  ....     23.0 

"  "      "  potassium         "  ....     39.0 

"  "      "  silver  "  ....    107.7 


ATOMIC    WEIGHT.  2IQ 

II. 

A  STUDY  OF  OXYGEN  AND  SOME  OF  ITS  COMPOUNDS. 

The  study  of  chlorine  and  its  affiliated  elements  has 
afforded  a  considerable  number  of  suggestions.  These 
may  well  be  applied  to  other  elements.  Oxygen  may 
well  be  studied  first,  because  of  its  great  importance  in 
this  as  well  as  other  relations. 

(a)  Experimental  Fact.  —  The  density  of  oxygen  gas 
is  about  16. 

Inference.  —  The  atomic  weight  of  oxygen  is  about  16. 

(b)  Experimental  Pact.  —  The  volume  composition  of 
water  vapor  is  as  follows  :  two  volumes  of  hydrogen  and 
one  volume  of  oxygen. 

Inference  I. — The  formula  of  water  is  H2O  (see  pp.  226  and-229). 
Inference  2.  —  The  molecular  weight  of  water  is  about  2  +  16  =  iS. 

(c)  Experimental  Fact.  —  The  vapor  density  of  water 
vapor  is  about  9. 

Inference.  —  The  molecular  weight  of  water  is  about  9x2  =  about  18. 
This  inference  is  based  on  results  obtained  under  chlorine.     It  sustains 
the  views  already  adopted  in  this  section. 

(d}  Experimental  Fact.  —  Gravimetric  analysis  shows 
that  water  is  made  up  as  follows  :  — 

Hydrogen    ....      ii.i  i  parts  by  weight. 
Oxygen        ....     88.88 
99-99 
The  ratios  are  as  i  :  8  or  2  :  16. 


220  ATOMIC  WEIGHT. 

Inference.  —  These  facts  add  support  to  the  views  previously  accepted, 
and  contribute  greatly  to  create  confidence  in  the  general  principles  as 
well  as  the  numerical  results  adopted. 

(e)  Experimental  Facts.  —  Oxygen  forms  compounds 
with  sodium,  potassium,  and  silver,  having  the  composi- 
tion given  in  the  following  table.  (As  a  matter  of  fact 
it  forms  many  others,  but  this  set  is  selected  as  afford- 
ing a  strict  continuity  to  the  argument.) 

EIGHTH   TABLE. 
PERCENTAGE  BASIS. 

Sodium  and  Oxygen.         Potassium  and  Oxygen.  Silver  and  Oxygen. 

Per  cents.  Per  cents.  Per  cents. 

Sodium,  74.19          Potassium,  82.98         Silver,  93-O9 

Oxygen,  25.81          Oxygen,  17.02         Oxygen,  6.91 


100.00  loo.oo  100.00 

If  now  the  results  of  the  eighth  table  are  computed 
on  another  basis,  i.e.,  using  the  atomic  weights  accepted 
for  sodium  23,  for  potassium  39,  and  for  silver  107.7, 
there  is  afforded  a  new  table.  Its  results  are  surprising, 
but  they  present  a  strict  statement  of  facts  —  in  a 
special  form  merely. 

NINTH  TABLE. 
ATOMIC  WEIGHT  BASIS. 

Sodium  and  Oxygen.         Potassium  and  Oxygen.          Silver  and  Oxygen. 
Sodium,  23          Potassium,  39         Silver,  IO7-7 

Oxygen,  8         Oxygen,  8         Oxygen,  8. 

31  47  "S-7 

Inference.  —  Either  the  atomic  weight  of  oxygen  is  8  instead  of  16;  or, 
if  it  is  indeed  16,  then  the  atomic  weights  accepted  for  sodium,  potassium, 


ATOMIC    WEIGHT.  221 

and  silver  are  only  one-half  what  they  should  be;  or  else  the  compounds 
have  the  formulas  Na.2O,  K2O,  Ag2O,  respectively.  This  latter  supposition 
satisfies  all  the  foregoing  facts  (and  many  others)  so  well,  that  it  has  been 
universally  adopted,  and  with  it  the  atomic  weight  16  (or  thereabouts)  for 
oxygen. 

On  this  view  the  following  table  may  be  arranged. 
It  is,  as  to  numerical  relations,  a  strict  statement  of 
experimental  facts. 

TENTH   TABLE. 

NEW  FORMULAS. 

Na^     23x2  =  46  K.2,     39X2=78  Ag2,     107.7x2=215.4 

O,  16  O,  16  O,  16. 

62  94  231.4 

Inferences  from  the  Tenth  Table.  —  From  various  facts  already  pre- 
sented, the  following  two  groups  of  formulas  have  been  accepted  :  — 

Hydrochloric  acid,  HC1  Hydrogen  oxide  (water),     H2O 

Hydrobromic  acid,  HBr 

Hydriodic  acid,  HI 

Sodic  chloride,  NaCl  Sodic  oxide,  Na2O 

Sodic  bromide,  NaBr 

Sodic  iodide,  Nal 

Potassic  chloride,  KC1  Potassic  oxide,  K^O 

Potassic  bromide,  KBr 

Potassic  iodide,  KI 

Argentic  chloride,  AgCl  Argentic  oxide,  Ag.2O 

Argentic  bromide,  AgBr 

Argentic  iodide,  Agl 

An  inspection  of  these  formulas  recalls  the  striking  and  very  important 
suggestion  that  oxygen  possesses  a  different  numerical  nature  from  chlo- 
rine, bromine,  and  iodine,  not  only  in  that  it  has  a  different  atomic  weight 
from  theirs,  but,  further,  in  this,  that  while  chlorine,  bromine,  and  iodine 
are  satisfied  to  combine  with  hydrogen,  sodium,  potassium,  and  silver,  atom 
for  atom,  the  atom  of  oxygen  is  only  satisfied  when  it  combines  with  two 
atoms  of  the  elements  in  question, 


222  ATOMIC    WEIGHT. 

This  fact  is  emphasized  by  a  great  many  other  compounds,  —  so  much 
so  that  oxygen  is  called  a  dyad,  and  the  other  elements  mentioned  are 
called  monads.  Further,  an  atom  of  oxygen  is  said  to  have  two  points  of 
attraction,  while  an  atom  of  hydrogen  (and  one  atom  of  each  of  the  other 
substances  mentioned)  is  said  to  have  one  point  of  attraction. 

Yet  further,  the  property  of  the  elementary  atoms  by  virtue  of  which 
they  attract  different  numbers  of  atoms  is  called  equivalence  or  valence. 
And  an  atom  of  hydrogen  is  accepted  as  the  unit  of  valence.  (See  p.  223.) 

III. 

A  STUDY  OF  SULPHUR  AND  SOME  OF  ITS  COMPOUNDS. 

Sulphur  may  be  discussed  very  much  as  oxygen  has 
been. 

(a)  Experimental  Pact.  —  The  density  of  sulphur  vapor 
is  found  to  be  about  32.2. 

Inference.  —  The  atomic  weight  of  sulphur  is  about  32. 

(b)  Experimental   Fact.  —  Sulphuretted  hydrogen  gas 
is  found  to  be  composed  of  hydrogen  and  sulphur,  and 
to  have  the  density  about  17.2. 

Inference  I .  —  Its  molecular  weight  is  34. 

Inference  2.  —  It  is  probably  made  up  of  one  atom  of  sulphur  weighing 
32,  and  two  atoms  of  hydrogen  weighing  2. 
Inference  3.  —  Its  formula  is  probably  H2S. 

(c)  Experimental   Fact.  —  Sulphuretted   hydrogen   gas 
has  been  found  to  have  the  percentage  composition  :  — 

Hydrogen    .     .     .       5.88  parts  by  weight. 
Sulphur  .     ...     94.12     "  " 

Total  ....   100. 

The  ratios  of  these  numbers  are  evidently  as  I  :  16, 
or  as  2  :  32. 


ATOMIC    WEIGHT.  223 

Inference.  —  These  facts  sustain  the  views  already  presented  as  to  the 
composition  of  sulphuretted  hydrogen,  and  that  the  atomic  weight  of 
sulphur  is  32. 

(d)  Experimental  Fact.  —  The  specific  heat  of  sulphur 
has  been  found  to  be  .188.  But  — ^  =  about  33. 

.  loo 

Inference.  —  This  affirms  the  selection  of  the  number  32  as  the  atomic 
weight  of  sulphur. 

Experimental  Fact.  —  The  most  exact  determinations 
of  the  atomic  weight  of  sulphur  have  been  based  upon 
its  combination  with  silver.  The  composition  of  sul- 
phide of  silver  has  been  learned  by  experiment  to  be  — 

Silver 87.07  per  cent. 

Sulphur 12:93        " 

100. 

Inference  I.  —  If  the  atomic  weight  previously  found  for  silver  is  107.7, 
then  the  atomic  weight  of  sulphur  is  16,  or  some  multiple  of  it.  But  the 
results  already  stated  suggest  the  number  32  as  the  proper  atomic  weight. 
If  this  view  is  accepted,  the  following  inference  may  be  obtained. 

Inference  2.  —  The  formula  of  sulphide  of  silver  is  Ag2S.     Then  — 

The  molecular  weight  of  Ag2  =  215.4    (87.07  per  cent) 
The  atomic  weight  of       S      =    32.       (12.93  Per  cent) 


247.4  (100.      per  cent) 

Inference  3.  —  From  these  results  sulphur  is  placed  in  the  class  of  dyads, 
as  oxygen  was.  The  notion  of  valence  already  reached  is  therefore  sus- 
tained by  the  studies  of  sulphur  above  described. 

Inference  4.  —  As  previously  intimated,  it  has  been  learned  that  certain 
elements  —  hydrogen,  chlorine,  bromine,  iodine,  sodium,  potassium,  silver 
—  have  the  equivalence  one,  and  certain  elements  —  oxygen,  sulphur  — 
have  the  equivalence  two.  The  suggestion  naturally  arises  that  perhaps 
other  elements  have  the  equivalence  three  or  four,  or  indeed  higher  nurn- 


224  ATOMIC    WEIGHT. 

bers  yet.  It  may  be  added  that  this  suggestion  is  amply  sustained  by 
facts,  some  of  which  will  be  presented  soon.  The  general  notion  of 
valence  is  adopted  as  a  fundamental  fact  of  chemistry. 

NOTE.  By  gravimetric  analysis  of  binary  compounds 
containing  on  the  one  hand  elements  whose  atomic 
weights  have  been  provisionally  adopted  as  just  de- 
scribed, and  on  the  other  hand  other  elements,  atomic 
weights  of  these  latter  elements  may  be  secured  —  sub- 
ject, of  course,  to  revision  in  the  light  of  additional  facts 
such  as  have  been  already  presented. 


CHAPTER    XXII. 
ATOMIC  WEIGHT  (continued}. 

FIFTH    STEP:    CONFIRMATION   OF  THE   ATOMIC 
WEIGHTS   CHOSEN. 

THE  preceding  chapters  on  atomic  weights  have  suf- 
ficed to  show  how  a  few  of  these  important  constants 
can  be  secured.  The  discussion  has  called  attention  to 
certain  methods  pursued  and  certain  precepts  accepted. 

Of  course,  when  the  atomic  weight  of  a  substance 
is  determined  by  a  sufficient  number  of  independent 
methods,  such  weight  may  be  used  in  fixing  the  molecu- 
lar formula  of  compounds  of  the  element. 

On  the  other  hand,  it  is  a  fact  of  deeper  significance 
that  ivhen  the  molecular  formula  of  a  compound  can  be 
determined  by  independent  methods,  this  formula  may 
be  of  great  service  in  securing  atomic  weights  of  ele- 
ments, or  in  confirming  those  already  secured.  Every 
effort  is  made,  therefore,  to  determine  molecular  formulas 
of  elements  and  compounds. 

I. 

MOLECULAR     FORMULA     SECURED     BY    VOLUME    COM- 
POSITION. 

In  illustration,  the  four  type-compounds  of  modern 
chemistry  may  be  referred  to  — 

225 


226  ATOMIC    WEIGHT. 

Hydrochloric  Acid.  —  First  Fact.  Hydrochloric  acid 
consists  of  hydrogen  and  chlorine,  and  nothing  else. 

Second  Fact.  —  Hydrochloric  acid  is  a  gas. 

Third  Fact.  —  Two  volumes  of  hydrochloric  acid  gas 
yield  by  decomposition  one  volume  of  hydrogen  and  one 
volume  of  chlorine. 

Inference.  —  The  molecule  of  hydrochloric  acid  has  some  one  of  the 
following  formulas :  — 

HC1, 

H2C12, 

H3C13, 


H«Cln. 

On  a  previous  page  certain  reasons  have  been  assigned  for  adopting  the 
formula  HC1.  Yet  other  reasons  will  be  assigned  hereafter.  (See  p.  228.) 

Fourth  Fact.  —  Hydrogen  and  chlorine  do  not  form 
any  compound  but  this  one,  called  hydrochloric  acid. 

Inference.  —  Hydrochloric  acid  is  the  simplest  possible  compound  of  the 
elements  hydrogen  and  chlorine.  Therefore  it  is  a  compound  containing 
one  atom  of  each. 

NOTE.  This  inference  is  not  a  particularly  valid  one,  but  it  naturally 
arises  in  cases  of  a  single  compound  of  two  elementary  substances. 

In  some  cases  it  is  distinctly  misleading.  Thus,  previous  to  the  recog- 
nition of  hydrogen  dioxide,  but  one  compound  of  hydrogen  and  oxygen 
was  known;  viz.  water.  On  this  general  ground  the  formula  HO  was 
adopted.  Subsequently  the  volume  relations  and  other  considerations 
(some  of  them  stated  in  this  chapter)  have  led  to  the  adoption  of  the 
formula  H2O. 

Water.  —  First  Fact.  Water  consists  of  hydrogen  and 
oxygen,  and  nothing  else. 

Second  Fact.  —  Water  may  be  changed  into  a  vapor 
and  experimented  upon  in  that  form. 

Third  Fact.  —  Two  volumes  of  water  vapor  yield,  by 


ATOMIC    WEIGHT.  22/ 

decomposition,  two  volumes  of  hydrogen  gas  and  one 
volume  of  oxygen  gas. 

Inference.  —  The  formula  of  water  is  probably  one  of  the  following :  — 
H20, 


But  reasons  have  been  heretofore  stated,  favoring   the  view  that  the 
Hereafter  other  reasons  will  be  stated  for  this  view. 


Ammonia  Gas.  —  First  Fact.  Ammonia  gas  consists 
of  hydrogen  and  nitrogen,  and  nothing  else. 

Second  Fact.  —  The  substance  is  a  gas. 

Third  Fact.  — When  two  volumes  of  ammonia  gas  are 
decomposed,  they  afford  one  volume  of  nitrogen  gas 
and  three  volumes  of  hydrogen  gas. 

Inference.  —  The  formula  for  ammonia  gas  is  one  of  the  following :  — 

H3N, 
H6N2, 

H9N3, 

H3llNB. 

The  simplest  formula,  H3N,  may  be  accepted  for  the  present,  with  the 
intention  of  changing  it  if  facts  hereafter  discovered  demand  such  change. 

Marsh  Gas.  —  First  Fact.  Marsh  gas  consists  of  car- 
bon and  hydrogen,  and  nothing  else. 

Second  Fact.  —  The  substance  is  a  gas. 

Third  Fact.  —  Two  volumes  of  marsh  gas  yield,  by 
decomposition,  four  volumes  of  hydrogen  gas.  (The 
volume  relations  of  the  carbon  cannot  be  stated,  since 
carbon  cannot  be  obtained  in  a  state  of  gas.) 


228  ATOMIC    WEIGHT. 

Inference,  —  The  formula  of  marsh  gas  is  one  of  the  following:  — 

H4C, 
H8C2, 

H12C3, 


H4nCn. 

The  simplest  formula  H4C  may  be  adopted  at  present,  with  the  inten- 
tion of  changing  it  if  facts  subsequently  discovered  demand  such  change. 

(Evidently  the  fact  that  carbon  is  not  obtainable  in  a  gaseous  form 
diminishes,  to  some  extent,  confidence  in  the  formula  adopted.) 


II. 

MOLECULAR     FORMULA     BASED     UPON    CHEMICAL    SUBSTI- 
TUTION. 

Hydrochloric  Acid.  —  First  Fact.  Many  chlorides  may 
be  formed  by  substituting  certain  metals  for  the  hydro- 
gen in  hydrochloric  acid  :  such  are  the  well-known  chlo- 
rides, sodium  chloride,  potassium  chloride,  silver  chloride. 

Second  Fact.  —  When  such  substitutions  as  those  just 
referred  to  are  made,  it  is  found  that  the  whole  of  the 
hydrogen  may  be  replaced  by  a  metal,  but  no  fractional 
part  can  be.  Thus  it  is  not  possible  to  form  a  chloride 
in  which  part  of  the  hydrogen  has  been  replaced  by 
potassium  and  part  of  the  hydrogen  remains  unreplaced. 
Replacement  must  be  of  the  whole  of  the  hydrogen  or 
of  none  at  all. 

Inference  I.  — The  amount  of  hydrogen  in  hydrochloric  acid  is  chem- 
ically indivisible.  In  other  words,  it  is  an  atom. 

NOTE.  It  is  true  that  the  chemist  cannot  so  carry  out  his  experiment 
as  to  work  upon  a  single  molecule  of  hydrochloric  acid.  The  smallest 
quantity  upon  which  he  can  experiment  must  necessarily  contain  millions 
of  molecules.  But  this  does  not  in  any  way  invalidate  the  conclusions 
just  reached. 

If  in  a  mass  of  hydrochloric  acid  containing  (say)  four  molecules  of 


ATOMIC    WEIGHT.  22Q 

hydrochloric  acid,  one-half  of  the  hydrogen  were  replaced  by  potassium, 
there  might  be  a  change  somewhat  as  indicated  below :  — 

First  Stage.  Second  Stage. 
HC1,  HC1, 

HC1,  HC1, 

HC1,  KC1, 

HC1.  KC1. 

In  a  sense,  however,  all  the  hydrogen  in  the  hydrochloric  acid  experi- 
mented upon  has  been  replaced  by  potassium.  But  the  formulas  given 
represent  simply  an  incomplete  operation.  They  do  not  represent  a 
new  and  single  compound  containing  part  hydrogen  and  part  potassium. 
Instead,  they  represent  a  mixture  of  compounds,  the  one  having  all  its 
hydrogen  replaced  by  potassium,  as  already  intimated,  the  other  having 
none  of  its  hydrogen  yet  replaced. 

By  a  process  of  experiment  and  reasoning  entirely  similar  to  the  fore- 
going, it  may  be  shown  that  the  chlorine  in  hydrochloric  acid  may  be 
replaced  by  another  element;  for  example,  bromine  or  iodine.  It  is  found, 
however,  that  bromine  and  iodine  respectively  replace  the  whole  of  the 
chlorine  in  the  hydrochloric  acid,  or  none  at  all.  They  cannot  replace 
the  one-half  or  one-fourth,  nor  any  other  fractional  part  of  the  chlorine. 

Inference  2. — The  chlorine  in  the  molecule  of  hydrochloric  acid  is 
practically  indivisible.  That  is,  it  is  a  single  atom. 

Thus  it  seems  to  be  proved  that  the  formula  for  hydrochloric  acid 
is  HC1. 

Water.  —  First  Fact.  Many  compounds  may  be  formed 
by  substituting  proper  metals  for  the  hydrogen  in  water. 
Thus  the  well-known  compounds,  potassic  oxide,  sodic 
oxide,  may  be  easily  formed. 

Second  Fact.  — When  such  substitutions  as  those  just 
referred  to  are  made,  it  is  found  that  they  may  be  accom- 
plished in  two  different  ways  ;  i.e.  either  the  whole  of  the 
hydrogen  may  be  replaced  by  the  potassium  and  sodium, 
respectively,  or  the  half  of  the  hydrogen  may  be  so 
replaced.  In  these  cases  two  entirely  different  substan- 
ces are  produced.  When  the  whole  of  the  hydrogen  is 


23O  ATOMIC    WEIGHT. 

replaced,  the  substance  called  potassic  oxide  is  produced. 
It  contains  78  parts  by  weight  of  potassium,  and  16  parts 
by  weight  of  oxygen.  When  the  half  of  hydrogen  is 
replaced,  a  distinct  and  well-known  compound  is  pro- 
duced, called  potassium  hydroxide.  It  contains  39  parts 
by  weight  of  potassium,  I  part  by  weight  of  hydrogen, 
1 6  parts  by  weight  of  oxygen. 

Here,  then,  it  is  seen  that  potassium  may  replace  the 
half  of  the  hydrogen  in  water  or  the  whole  of  it.  But 
the  displacement  of  hydrogen  in  water  is  not  possible 
in  any  other  fractional  zvay. 

By  a  course  of  experiment  and  reasoning  similar  to 
that  pursued  with  hydrochloric  acid,  it  may  be  shown 
that  the  oxygen  of  water  is  not  divisible;  in  other 
words,  is  a  single  atom. 

Inference  I. —  It  appears  that  the  one-half  of  the  hydrogen  that 
is  in  a  molecule  of  water  is  the  indivisible  portion  of  hydrogen;  i.e.  is  one 
atom.  In  other  words,  it  appears  that  the  molecule  of  water  has  two 
atoms  of  hydrogen. 

Inference  2.  —  It  appears  that  the  proper  formula  for  water  is    H2O. 

Ammonia  Gas.  —  By  the  use  of  undoubted  facts  of 
observation  and  a  method  of  reasoning  similar  to  that 
pursued  in  the  two  preceding  illustrations,  it  may  be 
shown  that  the  hydrogen  of  ammonia  gas  is  divisible 
into  three  parts,  and  that  its  nitrogen  is  not  divisible. 

Thus  the  inference  is  secured  that  the  formula  for  ammonia  gas  is  H3N. 

Marsh  Gas.  —  In  similar  fashion,  marsh  gas  may  be 
shown  to  have  an  amount  of  hydrogen  that  is  divisible 
into  four  parts  and  not  into  any  other  fractional  amounts, 
and  that  its  carbon  is  indivisible.  (See  p.  168.) 


ATOMIC    WEIGHT.  23! 

Thus  it  is  apparent  that  the  formula  H4C  should  be  adopted  for 
marsh  gas. 

The  foregoing  discussion  has  sustained  an  opinion  previously  expressed 
with  reference  to  the  valence  of  chlorine  and  oxygen;  viz.  chlorine  has 
been  made  out  to  be  a  monad,  oxygen  a  dyad,  and  so  also  nitrogen  and 
carbon  respectively  a  triad  and  a  tetrad. 

Many  other  facts  of  chemical  substitution  could  be  presented  to  sustain 
the  views  just  enunciated,  and  thence,  of  course,  to  sustain  the  formulas 
already  adopted. 

III. 

RELATION    OF    MOLECULAR    WEIGHT    TO    MELTING-POINTS 
OF    SOLIDS    AND    BOILING-POINTS    OF    LIQUIDS. 

Chemistry  as  a  system  cannot  be  complete  until  ele- 
ments and  compounds  can  be  arranged  in  orderly  lists, 
showing  regular  progression  of  the  various  physical  and 
chemical  properties  of  the  substances. 

Many  small  groups  are  recognized  where  such  a  prin- 
ciple as  this  is  successfully  carried  out. 

There  are  certain  marked  cases  of  the  simultaneous 
advance  of  molecular  weights  and  boiling-points  in  the 
case  of  elementary  substances.  For  example  note  the 
following  :  — 

Name.  Atomic  Weight.     Molecular  Weight.       Boiling-point. 

Chlorine,  35.4  70.8  -  33.60°  C. 

Bromine,  79.8  T59-6  59.27°  C. 

Iodine,  126.6  253.2  250°       C. 

Such  advance,  however,  is  by  no  means  uniform.  If  the  list  of  elemen- 
tary substances  be  arranged  in  an  order  commencing  with  that  of  lowest 
atomic  weight,  and  ending  with  that  of  highest  atomic  weight,  it  will  be 
found  that  while  there  is  a  general  tendency  toward  increase  of  melting- 
point  (for  most  of  the  elements  are  solid  at  ordinary  temperature),  this 
increase  is  by  no  means  uniform  or  even  regular. 


232  ATOMIC    WEIGHT. 

The  facts  with  respect  to  compound  substances  are  so  marked,  however, 
that  the  chances  are  that  the  order  of  arrangement  of  elements  by  atomic 
weights  is  not  quite  correct;  that  some  of  the  elements  in  the  solid  state 
may  have  a  larger  number  of  atoms  than  others;  and  so  the  proper  molec- 
ular weight  of  elements  (at  present  in  most  cases  unknown)  may  be  at 
present  recognized  only  in  those  few  that  can  be  given  in  regular  order  of 
melting-points. 

Here  is  a  group  showing  such  progress  :  — 

Name.  Formula.     Molecular  Weight.     Boiling-point. 

Sulphur  dioxide,  SO2  64  -  10°  C. 

Sulphur  trioxide,  SO3  80  46°  C. 

In  case,  however,  of  the  compounds  of  carbon  and 
hydrogen,  several  series  can  be  constructed  which  ad- 
vance with  very  striking  regularity  at  once  in  molecular 
weight  and  in  boiling-point. 

»r  zr          7  Molecular  Boiling- 

Name.  Formula.  „,  .   ,,  .    .    * 

Weight.  point. 

Methane  (marsh  gas),    CH4  16  (gas) 

Ethane,  C2H6  30  (gas) 

Propane,  C3H8  44  (gas) 

Butane,  C4H]0  58  i°     C. 

Pentane,  C5H12  72  38°     C. 

Hexane,  C6H14  86  71.5°  C. 

Heptane,  C7H16  100  98.4°  C. 

Octane,  C8H18  114  125.5°  C. 

A  Study  of  Certain  Nitrogen  Compounds.  —  One  use 
that  can  be  made  of  boiling-point  may  be  illustrated  by 
a  short  study  of  the  compounds  of  nitrogen  and  oxygen. 

These  compounds  are  as  follows :  — 

Nitrogen  monoxide,  N2O;  vapor  density,  21.99;  melting-point      •    —  99°  C. 

Nitrogen  dioxide,  N2O2  or  NO,  not  liquefied  at —  1 10°  C. 

Nitrogen  trioxide,  N2O3;   vapor  density,  37.95;   liquefies    .     .     .    —  10°  C. 


ATOMIC    WEIGHT.  233 

Nitrogen  tetroxide,  N2O4  or  NO2 ;  v.  d.  anomalous  {  solidifies  at         ~  9°  C. 

I  liquid  boils  at        22°  C. 

Nitrogen  pentoxide,  N2O5;   solid  melts  at 30°  C. 

The  vapor  density  for  nitrogen  monoxide  (about  22)  points  to  a  molec- 
ular weight  44,  and  this  corresponds  to  the  requirements  of  the  formula  N2O. 
The  vapor  density  for  nitrogen  trioxide  (about  38)  points  to  the  molecular 
weight  76.  This  corresponds  to  the  requirements  of  the  formula  N2O3. 

There  are  reasons  that  need  not  be  specified  here  for  adopting  for 
nitrogen  pentoxide  the  formula  N2O5. 

The  following  question  then  arises  with  respect  to  the 
two  compounds  remaining  :  — • 

Is  the  formula  N2O2  or  the  formula  NO  to  be  pre- 
ferred for  nitrogen  dioxide  ?  Comparing  the  boiling- 
points  of  this  compound  with  the  compound  N2O,  called 
nitrogen  protoxide,  it  is  seen  that  the  boiling-point  of 
N2O  is  very  much  lower.  In  accordance  with  a  general 
rule,  substances  having  lower  boiling-points  should  be  of 
simpler  constitution.  A  substance  having  the  formula 
N2O2  is  of  more  complex  constitution  than  a  substance 
having  the  formula  N2O.  If,  however,  we  assign  to  the 
substance  designated  as  N2O2  the  formula  NO,  its  con- 
stitution becomes  simpler  than  that  of  N2O  ;  it  then 
accords  with  the  general  rule. 

There  are  certain  facts  with  respect  to  the  substance 
designated  as  N2O4  that  lead  to  the  conclusion  that  this 
is  the  correct  formula  at  low  temperatures.  It  is  thought 
at  high  temperatures  it  dissociates,  forming  the  simpler 
molecule  NO2.  (See  p.  144.) 

NOTE.  It  appears  probable  that  the  following  general  law  may  be 
safely  accepted :  — 

General  Law.  — The  more  complex  compounds  of  a  series  condense 
more  easily  to  liquids  and  solids  and  decompose  more  readily  by  heat  than 
the  less  complex  compounds. 


234 


ATOMIC    WEIGHT. 


IV. 

RELATION    OF    MOLECULAR    FORMULA   TO    CRYSTALLINE 

FORM. 

Mitscherlich's  law  of  isomorphism  may  be  stated  as 
follows :  — 


FIG.  129.  —  Mass  of  alum  crystals. 

In  general,  when  two  solid  compounds  are  isomor- 
phous  —  that  is,  have  the  same  crystalline  form,  — they 
have  the  same  number  of  atoms  and  the  same  molecular 
arrangement. 

It  hardly  needs  mention  that  in  applying  the  law  it 
must  be  remembered  that  compound  radicles,  like  am- 
monium, methylamine,  ethylamine,  etc.,  must  be  counted 
as  elements. 


ATOMIC    WEIGHT. 


235 


It  must  be  admitted  that  there  are  well-recognized  cases  of  substances 
of  similar  crystalline  form  that  are  evidently  not  chemically  analogous; 
again,  substances  possessing  most  marked  chemical  resemblances  are 
known  that  solidify  in  differing  crystalline  forms.  But  while  there  are 
many  exceptions  to  the  law,  and  it  is  not  an  authoritative  guide,  yet  it  is 
of  occasional  value  in  confirming  results  obtained  by  other  methods. 

i.  The  law  is  well  illustrated  by  the  alums.  A  certain  substance  called 
alum  has  been  recognized  with  more  or  less  distinctness  for  at  least  two 
thousand  years.  During  the  last  century  its  chemical  composition  has 
been  distinctly  made  out.  It  is  usually  expressed  by  the  formula 

K2S04.A12(S04)3.24H.20. 


FIG.  130.  —  Diagrams  showing  different  forms  assumed  by  crystals  of  the  first  or  regular 

system. 

It  crystallizes  easily  and  distinctly  in  cubes  or  regular  octohedrons,  or  some 
simple  modification  of  these  belonging  to  the  first  or  regular  system. 

Within  a  few  years  at  least  a  dozen  substances  have  been  recognized 
which  bear  such  marked  structural  resemblance  to  ordinary  alum  that  they 
have  all  been  called  alums. 

Examples :  — 

Potassio-aluminic  alum  (ordinary  alum),        K2SO4  •  A12(SO4)3  •  24  H2O. 
Sodio-aluminic  alum,  Na2SO4  •  A12(SO4)3  •  24  II,O. 

Ammonio-aluminic  alum,  (NH4)2SO4  •  A12(SO4)3  •  24  H2O. 

Potassio-chromic  alum,  K2SO4  •  Cr2(SO4)3  •  24JH2O. 

Ammonio-ferric  alum.  (NH4)2SO4  •  Fe2(SO4)3  •  24  H2O. 

The  fact  that  these  all  crystallize  in  form  similar  to  ordinary  alum  leads 
chemists  to  confidently  adopt  for  them  the  same  general  molecular  formula 
as  that  assigned  to  ordinary  alum.  Yet,  further,  two  other  inferences  are 
drawn;  viz.  that  potassium,  sod'ium,  and  ammonium  are  radicles  of  analo- 


236  ATOMIC    WEIGHT. 

gous  character,  and  that  aluminium,  chromium,  and  iron  are  also  of  anal- 
ogous chemical  character. 

2.  The  substances  calcic  carbonate,  potassic  nitrate,  and  sodic  nitrate 
have  some  marked  crystalline  resemblances.  Thus  a  certain  form  of  calcic 
carbonate,  found  crystallized  in  nature  and  called  arragonite,  is  recognized 
as  having  the  same  crystalline  form  as  potassic  nitrate.  Again,  a  slightly 
different  form  of  calcic  carbonate,  though  of  the  same  chemical  composi- 
tion, but  found  crystallized  in  nature  in  the  form  called  calcspar,  has  a 
crystalline  form  similar  to  that  of  common  sodic  nitrate. 

Here,  then,  are  three  substances  closely  related  as  to  crystalline  forms. 
Analysis  has  shown  that  possible  formulas  for  these  substances  are  — 

CaC03, 
KNO3, 
NaNO3. 

The  crystalline  resemblances  then  favor  the  adoption  of  these  formulas. 
It  may  be  added  that  the  formula  CaCO3  leads  to  the  approval  of  the  num- 
ber 40  as  the  atomic  weight  of  calcium,  and  the  other  formulas  favor  the 
employment  of  the  numbers  23  and  39,  already  well  substantiated,  for 
sodium  and  potassium  respectively. 

V. 

RELATION     OF     MOLECULAR     FORMULA      TO      MOLECULAR 
STABILITY. 

The  question  has  arisen  whether  the  substance  known 
as  nitrogen  dioxide  should  have  the  formula  N2O2,  or 
the  formula  NO.  Facts  already  given,  under  the  head 
of  boiling-points,  indicate  pretty  clearly  that  the  formula 
should  be  NO.  Certain  facts  with  respect  to  the 
decomposition  of  this  substance  as  compared  with  the 
decomposition  of  the  substance  nitrogen  monoxide 
(N2O)  sustain  this  view. 

Thus  the  general  rule  is  that  substances  of  more  complex  composition 
possess  less  chemical  stability  than  substances  of  less  complex  composition. 
Now  when  a  piece  of  glowing  phosphorus  is  plunged  into  the  gas  nitrogen 
monoxide  (N2O),  it  readily  decomposes  the  molecule,  withdrawing  oxygen; 


ATOMIC    WEIGHT.  237 

and  the  phosphorus  continues  to  burn  by  combining  with  this  oxygen. 
But  when  burning  phosphorus  is  introduced  into  the  gas  called  nitrogen 
dioxide  (N2O2  or  NO),  it  is  extinguished;  that  is,  it  does  not  withdraw  the 
oxygen.  The  conclusion  is  that  the  molecule  of  nitrogen  dioxide  is  of 
greater  chemical  stability  than  the  molecule  N.2O,  and  therefore  is  of  simpler 
constitution  than  the  molecule  N2O.  This  favors  the  assumption  that  the 
formula  of  nitrogen  dioxide  should  be  taken  as  NO,  and  not  as  N2O2.  For 
if  the  formula  were  accepted  as  N2O2,  we  should  have  the  more  complex 
molecule,"  having  the  greater  chemical  stability  instead  of  the  less,  as  the 
general  law  demands.  (See  p.  232.) 

VI. 

MOLECULAR    FORMULAS    SUGGESTED    BY    RELATIONSHIP. 

The  substances  marsh  gas,  methyl  alcohol,  and  formic 
acid  are  naturally  related.  The  accepted  formulas  are 
as  follows :  — 

Marsh  gas     .     .     .     CH4, 

Methyl  alcohol  .     .     CH3OH, 

Formic  acid  .     .     .     HOCHO,  or  HCOOH. 

So  the  substances  ethylene,  ethyl  alcohol,  and  acetic 
acid  are  naturally  related.  The  accepted  formulas  for 
these  are  as  follows  :  — 

Ethylene      .     .     .     C2H4, 

Ethyl  alcohol    .     .     C,H5OH, 

Acetic  acid  .     .     .     HO(C2H3O),  or  CH3COOH. 

Now  the  formulas  chiefly  in  question  are  those  of  formic  acid  and 
acetic  acid.  But  the  fact  that  the  marsh  gas  and  methyl  alcohol,  whosrt 
formulas  are  established,  have  each  one  atom  of  carbon,  favors  the  assump- 
tion that  formic  acid  contains  one  atom  of  carbon,  and  therefore  that  it  has 
the  formula  assigned. 

Again,  the  fact  that  ethylene  and  ethyl  alcohol,  whose  formulas  are  well 
established,  have  each  two  atoms  of  carbon,  favors  the  assumption  that 
acetic  acid  has  two  atoms  of  carbon,  and  therefore  that  it  has  the  formula 
assigned. 


238  ATOMIC    WEIGHT. 


VII. 

MOLECULAR  FORMULA  SUGGESTED  BY  PRODUCTS  OF 
DECOMPOSITION. 

When  the  two  organic  compounds,  formic  acid  and 
acetic  acid,  are  made  to  combine  with  alkaline  sub- 
stances to  produce  respectively  formates  and  acetates, 
it  is  observed  that  46  parts  of  formic  acid  do  the  same 
work  as  60  parts  of  acetic  acid.  These  numbers  may 
then  be  taken  temporarily  as  the  molecular  weights  of 
the  two  substances.  Taken  in  conjunction  with  other 
facts  of  analysis,  the  following  statement  may  be  pre- 
pared :  — 

Formic  Acid.  — The  formula  HCOOH  corresponds  to 
molecular  weight  46. 

Acetic  Acid. — The  formula  CH3COOH  corresponds 
to  molecular  weight  60. 

But  analysis  has  shown  that  46  parts  of  formic  acid 
contain  12  parts  of  carbon,  and  that  60  parts  of  acetic 
acid  contain  24  parts  of  carbon. 

Next  consider  the  products  of  decomposition. 

Experiment  has  shown  that  when  these  acids  are  subjected  to  the  cur- 
rent of  the  galvanic  battery  they  are  decomposed.  Now  it  is  observed 
that  from  formic  acid  but  one  carbon  compound  is  produced,  i.e.  carbon 
dioxide.  But  from  acetic  acid  two  compounds  of  carbon  are  produced,  — 
carbon  monoxide  and  ethane.  These  facts  suggest  that  the  carbon  in 
formic  acid  acts  somehow  as  a  unit,  while  in  acetic  acid  there  is  such  a 
difference  in  the  condition  of  the  carbon  that  it  is  easily  susceptible  of 
division  into  at  least  two  parts,  the  one  part  doing  one  thing,  the  other 
part  doing  another  thing. 

These  facts  —  while  not  absolutely  conclusive  —  favor  the  opinion  that 
formic  acid  contains  one  atom  of  carbon,  while  acetic  acid  contains  two 
atoms  of  carbon. 


ATOMIC    WEIGHT.  239 

VIII. 

THE     ADOPTED     MOLECULAR     FORMULA     SUPPORTED     BY 
CERTAIN    EXCEPTIONAL    COMPOUNDS. 

Certain  double  salts,  produced  naturally  or  artificially, 
may  point  out  the  existence  of  definite  molecular 
groups. 

Thus  in  the  Solvay  soda  works,  at  Syracuse,  N.Y.,  a 
salt  was  artificially  formed  in  considerable  quantity  (al- 
though decomposable  by  water)  which  had  practically 
the  following  formula:  MgCO3  •  Na2CO3  •  NaCL  This 
combination  was  evidently  of  those  single  groups  which 
are  accepted  as  molecules.  Thus  it  seems  to  establish 
the  three  formulas  adopted  for  the  three  substances 
taking  part  in  it.1 

IX. 

OTHER  ILLUSTRATIONS  OF  THE  CLOSE  CONNECTION  BE- 
TWEEN THE  PROPERTIES  OF  SUBSTANCES  AND  THEIR 
MOLECULAR  WEIGHTS. 

(a)  Density    of    Liquids   as   related   to    their    Molecular 
Structure,  —  The  following  law,  called  Groshans's  law, 
has  been  enunciated  :  — 

At  the  temperature  of  ebullition,  the  density  of  compound  bodies  in  the 
liquid  state  is  in  proportion  to  the  number  of  the  atoms  in  their  molecules. 

(b)  The  Relation  of  the  Physiological  Action  of  Inorganic 
Compounds  with  their  Molecular  Weights.  —  In  a  study  of 
the  influence  of  different  salts  in  solution  when  intro- 
duced into  the  blood  of   living   animals,   it   has  been 
observed  that  the  acid  radicle  of  the  salt  has  but  little 

1  Chemical  News,  Ivii.  3. 


24O  ATOMIC    WEIGHT. 

influence.  Any  physiological  action  produced  depends 
almost  entirely  on  the  electro-positive  component  of  the 
salt,  i.e.  upon  the  metal.  Again,  it  has  been  noted  that 
practically  all  the  elements  found  in  organized  bodies 
have  atomic  weights  of  less  than  40.  Thus  it  appears 
that  all  the  positive  elements  among  them  are  monads 
and  dyads.  Now,  it  has  been  noted  that  the  physio- 
logical action  of  substances  increases  from  the  monads 
onward. 

Dr.  J.  Blake,  who  has  studied  this  subject,  points  out  that  the  monads 
tend  to  affect  but  one  set  of  tissues  or  organs,  —  the  pulmonary  arteries. 
The  dyads  affect  two  or  more,  —  i.e.  the  centres  of  vomiting,  the  voluntary 
and  cardiac  muscles,  —  while  with  elements  of  higher  equivalence  the  influ- 
ence is  more  widespread  and  therefore  more  considerable  :  it  extends  to  the 
ganglia  and  even  the  brain  itself.  Again,  experiments  seem  to  show  that 
the  physiological  efficiency  of  substances  belonging  to  one  and  the  same 
isomorphous  group  is  directly  proportional  to  the  atomic  weights;  i.e. 
the  higher  the  atomic  weight,  the  greater  the  action.  The  law  has  been 
studied  with  respect  to  the  following  substances :  first,  lithium,  sodium, 
rubidium,  thallium,  silver;  second,  magnesium,  iron,  manganese,  cobalt, 
nickel,  copper,  zinc,  cadmium;  third,  calcium,  strontium,  barium,  lead; 
fotirth,  palladium,  platinum,  osmium,  gold. 

In  case  of  chlorine,  bromine,  and  iodine,  however,  the  action  seems  to 
vary  inversely  as  the  atomic  weights. 

In  case  of  potassium  and  ammonium  the  influence  is  also  partially 
exceptional. 

The  whole  study  is  an  important  one  and  may  be  looked  upon  as  likely 
to  offer  more  valuable  information  in  the  future.  Thus  it  may  be  that  the 
biological  relations  of  chemical  substances  may  assist  in  determining  the 
position  of  atoms  and  molecules  in  the  chemical  scale.1 

(c)  The  Magnetic  Rotary  Polarization  of  Compounds  as 
related  to  their  Chemical  Constitution. —  Chemists  have 
long  felt  assured  that  such  rotation  was  dependent 

1  Blake,  J.,  Chemical  News,  xliii.  191 ;  xlv.  in  ;  Ivii.  194. 


ATOMIC    WEIGHT. 


241 


upon  th-i  kind  of  molecules  involved  as  well  as  their 
number,  yet  difficulties  in  the  way  of  demonstration 
have  prevented  the  attainment  of  any  satisfactory  con- 
clusions. 

Dr.  Perk  in  has  studied  this  subject  very  carefully.  He  has  adopted  a 
new  system  of  unit  lengths  for  those  portions  of  the  substances  experi- 
mented upon;  that  is,  he  has  employed  such  portions  of  liquid  compounds 
as  would  produce  unit  lengths  of  columns  of  vapor  when  in  the  latter  con- 
dition. As  a  result,  it  appears  that  certain  definite  relations  do  exist 
between  this  magnetic  rotary  power  and  the  molecular  constitution  of 
bodies.  It  is  not  practicable  to  express  these  results  in  a  few  words. 
Reference,  therefore,  must  be  made  to  Perkin's  original  paper.1 

(d)  Freezing-points  of  Solutions  as  related  to  the 
Molecular  Weights  of  the  Substances  dissolved  :  Raoult's 
Method.  —  This  method  is  of  chief  importance  in  case 
of  compounds  which  cannot  be  vaporized  without  dis- 
sociation. It  is  based  upon  a  principle  described  by 
Coppet,  that  "salts  of  analogous  constitution,  dissolved 
in  quantities  proportional  to  their  molecular  weights, 
produce  in  their  solution  the  same  depression  of  the 
freezing-point  of  the  solvent." 

Raoult  has  made  a  careful  study  of  this  subject,  and  as  a  result  of  his 
work  the  law  seems  now  to  meet  general  acceptance. 

If  C  represents  the  depression  in  degrees  centigrade,  P  the  number  of 
grammes  of  substance  dissolved  in  100  grammes  of  water,  and  M  the 
molecular  weight  of  the  substance,  — 

—  =  A,  the  coefficient  of  depression  of  the  substance; 
MA  =  7\  the  molecular  depression  of  the  solvent. 

The  constant  T  varies  according  to  the  substances  used,  and  according 
to  the  solvents  employed.  Of  the  latter,  water,  benzene,  and  acetic  acid 
have  been  found  most  useful. 

i  Perkin,  W.  H.,  Journal  of  the  London  Chemical  Society,  1884,  Transac- 
tions, p.  421. 


242  ATOMIC    WEIGHT. 

This  method  is  of  especial  value  in  its  application  to  many  organic 
compounds  of  high  molecular  weight. 

One  general  result  of  Raoult's  studies  may  be  presented  in  the  follow- 
ing general  form :  — 

While  the  freezing-point  of  a  pure  liquid  is  constant,  every  molecule  of 
foreign  matter  that  dissolves  occasions  the  same  constant  depression  of  that 
point.  In  other  words,  in  dilute  solutions  the  depression  of  the  freezing- 
point  of  the  solvent  varies  with  the  ratio  between  the  numbers  of  molecules 
of  solvent  and  numbers  of  molecules  of  substance  dissolved. 

As  respects  vapor  tension  of  the  liquid,  Raoult  has  shown  that  its 
depression  bears  a  relation  to  the  percentage  of  foreign  molecules  which  is 
independent  of  temperature,  but  if  certain  proper  distinctions  are  made,  is 
such  that  it  represents  a  relative  depression  which  may  become  a  definite 
constant  for  any  substance  which  that  particular  liquid  may  dissolve. 

Raoult's  method  has  been  applied  to  the  determination  of  the  molecular 
weights  of  certain  of  the  carbohydrates.  As  a  result,  the  molecular  weight 
of  dextrin  was  found  to  correspond  approximately  to  the  number  6480. 
This  points  to  the  formula  20  (C12H20O]0).  In  similar  fashion  the  formula 
for  soluble  starch  has  been  suggested  to  be  five  times  this  value,  or 
5  (C12H20O10)20.  This  would  give  as  a  molecular  weight  of  soluble  starch 
about  the  number  32,4OO.1 

1  American  Chemical  Journal,  xi.  67 ;  also  xii.  130  and  142.  Chemical 
News,  Ix.  66. 


CHAPTER    XXIII. 
ATOMIC  WEIGHT  (continued). 

SIXTH    STEP:     BRING    ALL    THE    ATOMIC    WEIGHTS     INTO 
ONE  TABULAR   STATEMENT.     THE   PERIODIC   LAW. 

WHEN  the  atomic  weights  of  practically  all  the  ele- 
ments have  been  provisionally  adopted,  by  methods  in- 
volving the  principles  already  referred  to  (and  perhaps 
some  others),  then  the  "periodic  law"  may  be  consid- 
ered. By  its  use  certain  numbers  already  adopted  may 
be  confirmed ;  perhaps,  on  the  other  hand,  it  may  deter- 
mine a  new  selection  from  the  various  possible  atomic 
weights. 

The  Work  of  Newlands  and  of  Mendeleeff.  —  The  Eng- 
lish chemist  Newlands  (in  1863—64)  and  the  Russian 
chemist  Mendeleeff  (in  1869-70)  independently  pub- 
lished tables  of  the  elements  known  at  about  those 
dates.  They  first  arranged  all  known  elements  in  a 
long  list,  commencing  with  that  of  the  lowest  atomic 
weight,  and  advancing  numerically  to  that  of  the  high- 
est. In  considering  this  list,  they  noticed  that  the 
elements  formed  several  natural  series,  the  members  of 
a  given  series  showing  a  periodic  progress  in  chemical 
properties ;  for  example,  in  the  kind  and  amount  of 

243 


244  ATOMIC    WEIGHT. 

atom-fixing  power.  Upon  arranging  the  several  series 
one  above  another,  it  at  once  appeared  that  the  corre- 
sponding members  of  the  several  series  formed  natural 
groups.  For  example,  calcium,  strontium,  and  barium 
appeared  in  one  group  ;  so  also  did  phosphorus,  arsenic, 
and  antimony ;  and  also  sulphur,  selenium,  and  tel- 
lurium ;  and  also  chlorine,  bromine,  and  iodine. 

Professor  Crookes  remarks  :  "  Undoubtedly  one  of 
the  grandest  steps  taken  in  pure  chemistry  within  our 
epoch  has  been  the  discovery  of  the  periodic  law.  This 
generalization  (as  reference  to  Chemical  News  will  show, 
vols.  vii.,  x.,  xii.,  and  xiii.)  was  in  the  first  place  due  to 
Mr.  J.  A.  R.  Newlands.  It  was  some  time  afterwards  in- 
dependently discovered  by  Mendeleeff,  and  since  been 
developed  both  by  that  eminent  savant  and  by  Meyer 
and  Carnelley." 

The  total  result  of  the  Mendeleeff  classification  is 
now  known  as  "the  periodic  system."  It  is  presented 
in  the  table  (including  68  elements)  found  on  the  next 
page. 

As  soon  as  the  periodic  table  was  adjusted,  it  sug- 
gested the  important  truth  that  "the  properties  of  an 
element  are  a  periodic  function  of  its  atomic  weight." 
It  now  appears,  therefore,  that  when  a  new  element  is 
studied  and  its  properties  are  learned,  these  properties 
determine  its  place  in  the  periodic  table.  But  the  place 
in  the  table  at  once  suggests  which,  among  several  mul- 
tiples, shall  be  accepted  as  the  atomic  weight. 

The  system  has  now  secured  very  general  adoption.  Some  of  its  merits 
are  the  following :  — 

FIRST.  It  is  based  on  the  atomic  weights,  —  constants  which  it  must 
be  assumed  are  dependent  upon  some  fundamental  characteristics  of 
elements. 


ATOMIC    WEIGHT. 


245 


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246  ATOMIC    WEIGHT. 

SECOND.  It  throws  most  of  the  elements  into  groups  and  series  which 
accord  with  many  of  their  undoubted  geological,  physical,  and  chemical 
properties. 

It  cannot  be  denied  that  in  this  system  some  elements  are  brought 
together  that  do  not  appear  to  be  closely  related.  This  is  merely  equiva- 
lent to  admitting  that  the  system  is  not  yet  perfect.  Probably,  also,  some 
so-called  elements  are  wrongly  placed  because  of  their  peculiar  compound 
nature. 

THIRD.  It  helps  in  the  decision  as  to  which  of  several  combining  num- 
bers of  an  element  shall  be  accepted  as  its  atomic  weight.  Thus  indium 
might  have  the  number  75.6  or  the  number  113.4  (one  and  a-half  times 
the  former).  The  latter  number  is  now  chosen  under  guidance  of  the 
periodic  law. 

P^OURTH.  The  periodic  table  has  shown  some  gaps  in  the  series  of 
numbers  representing  atomic  weights.  On  this  basis  —  as  long  ago  as 
1871  —  Mendeleeff  predicted  the  existence  of  two  new  elements,  and  more, 
he  stated  their  general  range  of  properties.  To  one  he  gave  the  provisional 
name  eka-aluminium.  Now  in  1876  the  element  gallium  was  discovered, 
and  it  proved  to  be  the  predicted  eka-aluminium.  So  scandium,  discovered 
in  1879,  proved  to  be  Mendeleeff's  predicted  eka-boron. 

The  recently  discovered  element  samarium  fell  into  a  place  not  previ- 
ously occupied,  thus  contributing  to  support  the  system. 


Algebraic  Expression  of  the  Periodic  Law.  —  Professor 
Carnelley  has  recently  studied  the  periodic  law  with  a 
view  to  expressing  its  numerical  relations  in  the  form 
of  an  algebraic  formula.  For  reasons  which  are  given 
in  detail  in  the  memoir,  an  expression  of  the  form 

A  =  c  (m  +  Vz>) 

is  adopted,  where  A  represents  the  atomic  weight  of 
the  element ;  c,  a  constant  ;  m,  a  member  of  a  series  in 
arithmetical  progression,  depending  upon  the  horizontal 
series  in  the  periodic  table  to  which  the  element  be- 
longs ;  and  v,  the  maximum  valence,  or  the  number  of 
the  vertical  group  of  which  the  element  is  a  member. 


ATOMIC    WEIGHT.  247 

From  a  number  of  approximations  Professor  Carnelley  finds  that  m  is 
best  represented  by  the  value  o  in  the  lithium-beryllium-boron,  etc.,  hori- 
zontal series;  by  2],  in  the  sodium  series;  5,  in  the  potassium  series;  and 
8%  12,  15',  19,  22],  etc.,  in  the  subsequent  series.  Thus  m  is  a  member 
of  an  arithmetical  series  of  which  the  common  difference  is  2}  for  the  first 
three  members  and  3]  for  all  the  rest.  On  calculating  the  values  of  the 
constant  c  from  the  equation 

A 


for  55  of  the  elements,  the  numbers  are  all  found  to  lie  between  6.0  and 
7.2,  with  a  mean  value  of  6.6.  In  by  far  the  majority  of  cases  the  value  is 
much  closer  to  the  mean  6.6  than  is  represented  by  the  two  extreme  limits; 
thus  in  35  cases  the  values  lie  between  6.45  and  6.75.  If  the  number  6.6, 
therefore,  is  adopted  as  the  value  of  <r,  and  the  atomic  weights  of  the  ele- 
ments are  then  calculated  from  the  formula 


the  calculated  atomic  weights  thus  obtained  approximate  much  more  closely 
to  the  experimental  atomic  weights  than  do  the  numbers  derived  from  an 
application  of  the  atomic  heat  approximation  of  Dulong  and  Petit.  The 
number  6.6  at  once  strikes  one  as  being  remarkably  near  to  the  celebrated 
6.4  of  Dulong  and  Petit,  and  Professor  Carnelley  draws  the  conclusion  that 
there  must  be  a  connection  between  the  two.  This  assumption  appears  to 
be  supported  by  several  interesting  facts.1 

Prout's  Hypothesis.  —  -As  early  as  1816  the  theory  was 
suggested  that  the  atomic,  weights  may  be  represented 
by  numbers  that  are  exact  multiples  of  that  of  hydro- 
gen. This  led  to  the  further  suggestion  that  possibly 
hydrogen  is  a  sort  of  "  primordial  matter  which  forms 
the  other  elements  by  successive  condensations  of  it- 
self." 

The  most  critical  determinations  of  the  atomic  weights 
seem  at  present  to  afford  numbers  that  are  not  integral 

1  Nature,  xli.  304. 


248  ATOMIC    WEIGHT. 

multiples  of  that  for  hydrogen.  But  in  most  cases  the 
variations  are  but  slight,  and  it  cannot  be  declared  with 
certainty  that  the  atomic  weights  at  present  held  may 
not  be  subject  to  corrections  such  as  will  in  future 
afford  numbers  sustaining  Prout's  proposition. 

Thus  recent  and  careful  recalculations  of  the  atomic  weights  show  that 
"  thirty-nine  out  of  sixty-five  elements  have  weights  varying  each  by  less 
than  the  tenth  of  a  unit  from  even  multiples  of  the  atomic  weight  of 
hydrogen."  Of  the  remaining  elements,  twenty-six  have  atomic  weights 
that  are  known  to  be  defectively  determined.  Thus  Prout's  hypothesis 
acquires  new  interest. 


CHAPTER   XXIV. 

ATOMIC  WEIGHT  (continued}. 

ELEMENTARY   SUBSTANCES  AS   MOLECULAR. 

IT  is  believed  that  in  most  cases  elementary  sub- 
stances are  arranged  in  groups  which  may  be  properly 
called  molecules. 

FIRST.  This  view  is  sustained  by  the  volume  condi- 
tions of  certain  chemical  unions. 

Example  a.  —  Two  volumes  of  hydrogen  and  two 
volumes  of  chlorine  combine  to  form  four  volumes  of 
hydrochloric  acid  gas.  In  accordance  with  Avogadro's 
law,  two  volumes  of  hydrogen  may  be  considered  as 
having  n  molecules  of  hydrogen,  and  two  volumes  of 
chlorine  n  molecules  of  chlorine.  Then  four  volumes 
of  hydrochloric  acid  gas  must  have  2n  molecules  of  the 
substance  represented  by  HC1.  But  each  of  these  2n 
molecules  contains  at  least  one  atom  of  hydrogen,  a 
total  of  2n  atoms  of  hydrogen.  But  these  2n  atoms 
of  hydrogen  are  derived  from  n  molecules  of  hydrogen. 
Therefore  each  molecule  of  hydrogen  has  two  atoms  of 
hydrogen.  By  a  similar  process  of  reasoning  it  is  appar- 
ent that  each  molecule  of  chlorine  contains  two  atoms 
of  chlorine. 

In  certain  other  cases,  elements  may  be  shown  to  contain  four  atoms  in 
the  molecule. 

In  some  cases,  it  is  true,  the  number  of  atoms  in  a  molecule  cannot  be 

readily  determined. 

249 


2$o  ATOMIC  WEIGHT. 

Example  b.  —  When  sulphur  burns  in  oxygen  gas  to  form  the  molecule 
SO2,  there  is  neither  increase  nor  decrease  of  volume;  i.e.  apparently  one 
molecule  of  oxygen  gas  has  furnished  one  molecule  of  sulphur  dioxide,  SO2. 
This  favors  the  theory  that  the  amount  of  oxygen  gas  in  the  molecule  SO2 
is  the  same  as  the  amount  of  oxygen  gas  in  one  molecule  of  oxygen. 
Whence  one  molecule  of  oxygen  gas  appears  to  consist  of  two  atoms. 

When  carbon  burns  in  oxygen  gas  to  form  the  molecule  CO2,  there  is 
neither  increase  nor  decrease  in  volume.  By  a  course  of  reasoning  similar 
to  that  just  employed  with  respect  to  sulphur  dioxide,  the  view  then  brought 
forward  is  sustained  by  the  burning  of  carbon. 

SECOND.  This  view  harmonizes  with  certain  facts  of 
thermo-chem  istry. 

Example  a.  —  Phosphorus  burns  in  the  gas  nitrogen 
monoxide,  N2O.  It  also  burns  in  oxygen  gas.  Carbon, 
also,  will  burn  in  either  of  these  gases.  Now  it  is  ob- 
served that  more  heat  is  evolved  when  the  combustible 
burns  in  nitrogen  monoxide  than  when  it  burns  in 
oxygen. 

According  to  the  molecular  theory,  oxygen  has  mole- 
cules as  well  as  nitrogen  monoxide.  The  heat  afforded 
by  the  combustion  represents  a  difference  between  the 
true  amount  of  heat  evolved  by  the  chemical  action  and 
the  amount  of  heat  absorbed  in  decomposing  the  mole- 
cule containing  oxygen.  The  less  amount  of  heat  given 
off  by  the  combustion  in  pure  oxygen  suggests  that 
when  the  phosphorus  withdraws  an  atom  of  oxygen 
from  its  companion  atom,  more  energy  is  expended  than 
when  it  draws  the  oxygen  from  the  companion  nitrogen. 

Example  b.  —  When  the  very  explosive  compound  called  chloride  of 
nitrogen  decomposes,  the  reaction  is  as  follows :  — 

2  NC13  exploded  =  N2  +  3  C12. 

Now  when  this  operation  goes  on,  38,100  units  of  heat  are  liberated.  This 
extraordinary  result  is  believed  to  be  explainable,  on  the  molecular  theory, 
as  due  to  the  fact  that  the  affinity  of  an  atom  of  nitrogen  for  another  atom 


ATOMIC    WEIGHT. 

of  nitrogen,  and  the  affinity  of  an  atom  of  chlorine  for  another  atom  of 
chlorine,  are  far  greater  than  is  the  affinity  of  the  atom  of  nitrogen  for  the 
atoms  of  chlorine  with  which  it  was  combined  in  the  explosive  substance. 

It  is  true  that  nitrogen  as  an  elementary  gas,  and  under  certain  other 
circumstances,  is  very  inert.  But  nitrogen  is  found  as  a  constituent  of  a 
very  large  number  of  compounds.  Some  of  these  are  very  stable,  and  the 
nitrogen  holds  to  the  other  constituents  with  great  affinity.  Ammonia  gas 
(NH3)  is  an  example.  This  shows  no  fundamental  lack  of  affinity  in  the 
nitrogen  atom.  Perhaps  the  very  inertness  of  the  free  nitrogen  molecule 
may  be  due  to  the  affinity  binding  the  two  atoms  of  this  molecule  together. 

THIRD.  The  facts  of  allotropism  accord  with  this 
theory. 

Some  elementary  substances  are  capable  of  undergoing 
a  great  change  in  their  properties  without  any  change 
of  weight,  and  without  any  addition  or  withdrawal  of 
other  kinds  of  matter.  Thus  a  given  portion  of  ordi- 
nary oxygen  may  be  changed  —  at  least  partly  —  into 
ozone  and  back  again.  So  ordinary  phosphorus  may 
be  changed  into  red  phosphorus  and  back  again.  One 
of  the  forms  of  such  substances  is  called  the  allotropic 
form  of  it,  and  this  general  property  of  bodies  is  termed 
allotropism.  The  view  now  held  is  that  such  modifica- 
tions represent  some  change  in  the  number  of  atoms  in 
the  molecule  of  the  element,  or  some  change  in  their 
arrangement  within  the  molecule. 

Occasionally  the  term  allotropism  is  applied  to  compounds  to  describe  a 
kind  of  physical  isomerism.  Indeed,  the  three  states  of  aggregation  in 
which  a  compound  may  exist  may  be  considered  as  representing  a  sort  of 
allotropism. 

Many  natural  minerals  display  a  kind  of  allotropism  in  their  differences 
from  the  same  substances  artificially  produced. 

The  properties  of  ozone  support  the  theory  in  ques- 
tion. When  oxygen  changes  to  ozone,  condensation 
occurs  without  loss  of  matter;  and  when  ozone  changes 


2  $2  ATOMIC    WEIGHT. 

to  oxygen,  corresponding  expansion  occurs.  Ozone  is 
i^  times  as  heavy  as  oxygen  ;  in  other  words,  ozone 
has  the  density  24,  and  oxygen  has  the  density  16. 
According  to  Avogadro's  law,  and  the  molecular  theory 
of  elements,  ozone  should  have  the  molecular  weight  48, 
and  oxygen  the  molecular  weight  32.  This  means,  then, 
that  the  molecule  of  ozone  should  have  three  atoms  of 
oxygen,  and  a  molecule  of  ordinary  oxygen  should  have 
two  atoms  of  oxygen. 

Now  ozone  is  characterized  by  two  striking  properties  at  first  incon- 
sistent; i.e.  it  readily  oxidizes  certain  substances,  like  metallic  silver,  add- 
ing oxygen  to  the  silver.  Again,  it  readily  reduces  certain  substances  like 
oxide  of  silver;  i.e.  withdrawing  oxygen  from  the  silver.  But  the  incon- 
sistency is  immediately  explained  by  the  molecular  theory.  Ozone  (O3), 
by  imparting  oxygen  to  silver,  is  itself  changed  to  the  more  stable  form 
O.2;  i.e.  ordinary  oxygen.  Again,  ozone  (O3),  in  withdrawing  oxygen 
from  oxide  of  silver,  at  once  gives  rise  to  2  O2 ;  i.e.  to  two  molecules  of  the 
stable  form  of  oxygen  called  ordinary  oxygen. 

FOURTH.  This  theory  affords  the  best  explanation 
of  the  properties  of  certain  compounds  like  hydrogen 
dioxide  (H2O2).  Like  ozone,  it  readily  withdraws  oxygen 
from  certain  substances  and  it  readily  adds  oxygen  to 
certain  substances.  The  apparent  inconsistency  is  at 
once  explained  by  the  molecular  theory.  By  adding 
oxygen  to  certain  substances  the  unstable  molecule 
H2O2  is  changed  to  the  stable  molecule  H2O.  On  the 
other  hand,  in  withdrawing  oxygen  from  certain  sub- 
stances, one  atom  of  oxygen  withdrawn  unites  with  one 
atom  of  oxygen  from  the  H2O2,  thus  producing  the  sta- 
ble molecule  O2.  But  the  existence  of  such  a  molecule 
as  O2  is  the  very  point  that  this  discussion  is  now 
intended  to  support. 

FIFTH.     The  superior  activity  of  substances  in  the 


ATOMIC    WEIGHT.  253 

nascent  state  is  best  explained  by  this  theory.  It  is 
supposed  that  atoms  of  a  gas  when  freshly  liberated,  — 
in  other  words,  when  in  the  nascent  state,  —  have  not  yet 
combined  to  form  molecules  of  that  particular  element. 
They  are  then  more  ready  to  combine  with  other  sub- 
stances than  when,  as  in  their  ordinary  condition,  they 
have  to  leave  companion  atoms  to  do  so. 

SIXTH.  The  theory  accords  with  the  facts  enunciated 
in  the  law  of  Charles. 

If  the  expansion  of  compound  gases  by  increase  of  heat 
is  due  to  a  forcing  apart  of  a  certain  number  of  mole- 
cules, the  fact  that  the  elementary  gases  obey  the  same 
law  shows  that  they  must  have  molecules  of  the  same 
size  as  those  of  compound  gases. 

If  elementary  gaseous  substances  were  composed  of 
single  atoms  of  smaller  size,  they  should  be  expected  to 
expand  at  least  twice  as  much  as  compound  gases  by 
accession  of  heat. 

It  must  not  be  supposed  that  all  the  substances  known 
to  chemists  afford  satisfactory  information  as  to  their 
molecular  structure.  This  is  true  of  elementary  sub- 
stances, and  of  compound  substances  as  well.  Chemists 
are  still  in  the  dark  with  respect  to  the  proper  molec- 
ular formulas  of  certain  elements  and  compounds.  For 
the  present,  molecular  formulas  as  well  as  atomic  weights 
are  adopted  that  are  recognized  as  only  approximate,  and 
that  are  likely  to  demand  revision  hereafter. 


INDEX. 


[THE  NUMBERS  REFER  TO  PAGES.] 


Acid,  acetic,  237,  238. 

formic,  237,  238. 

hydrochloric,  77,  226,  228. 
Adhesion  between  gases  and  gases,  136. 

between  liquids  and  gases,  134. 

between  liquids  and  liquids,  131. 

between  solids  and  gases,  130. 

between  solids  and  liquids,  114. 

between  solids  and  solids,  114. 
Affinity,  chemical,  3,  141. 
Air,  atmospheric,  139. 
Allotropism,  251. 
Alloys,  128. 

fusible,  49. 
Alum,  126. 

crystalline  form  of,  235. 
Amalgams,  129. 
Ammonia  fountain,  134. 

gas,  227,  230. 

gas,  liquefaction  of,  59. 

gas,  thermolysis  of,  146. 
Ammonium,  27. 

Ampere,  A.  M.,  78,  80,  188,  190. 
Analysis,  26. 

Andrews,  Thos.,  20,  35,  58. 
Antiseptics,  165. 
Arragonite,  236. 
Atmosphere,  terrestrial,  137. 
Atoms,  3,  14,  15,  29,  30,  31,  32. 
Attraction,  chemical,  168,  170, 187, 188,  l 
Avogadro,  12,  78. 

Bacteria,  156,  158,  165,  166. 

Balance,  7,  12,  138. 

Barium,  3. 

Barometer,  8,  9,  10,  69. 

Becquerel,  188. 

Benzol,  32. 

Berthelot,  M.,  176,  177. 

Berthollet,  188. 

Berzelius,  J.  J.,  12,  188,  194,  195,  200. 

Bismuth  crystals,  108. 

Botany,  i. 

Boyle,  63,  64,  65,  67. 

Brodie,  29. 

Bromine,  217,  231. 

Bunsen,  ice  calorimeter,  214. 

Cadmium,  3. 
Cailletet's  apparatus,  61. 

254 


Calcspar,  236. 
Calorimetry,  178,  181. 
Calory,  177. 
Cane  sugar,  124. 
Capillarity,  115,  116. 
Carbon,  216. 

dioxide,  139. 

Carnelley,  Thos.,  29,  244,  246. 
Carre,  ice-machine,  51. 
Cathctometer,  69. 
Charles,  J.  A.  C.,  65,  66,  68. 
Chemistry,  2. 
Chlorine,  216,  231. 
Cleavage,  103. 
Cohesion,  101,  102. 
Colloids,  134. 

Compound  radicles,  27,  34. 
Compounds,  specific  heat  of,  215. 
Cooke,  J.  P.,  194. 
Corn  starch,  153. 
Corrosive  sublimate,  165. 
Critical  point,  58,  63,  64. 
Crookes,  Wm.,  30,  38,  39,  41,  194,  24 
Cryohydrates,  128. 
Crystals,  IOKJ,  no. 
Crystallization,  103,  104,  108. 

Dalton,  John,  6,  29,  196,  200. 
Davy,  Sir  H.,  188,  189. 
Deleuil,  apparatus,  58. 
Deliquescence,  122. 
Deville,  H.  St.  C.,  145. 
Dialysis,  132,  133. 
Diathermancy,  19. 
Diffusipmeter,  68. 
Dissociation,  49,  50. 
Dobereiner's  lamp,  131. 
Dulong,  213. 
Dumas,  J.  B.  A.,  12,  188. 

Efflorescence,  128. 
Eka-aluminium,  246. 
Eka-boron,  246. 
Electricity,  93,  94,  96,  97. 

related  to  chemical  change,  147. 
Electric  arc,  96,  147. 

spark,  148. 
Electrolysis,  24,  147. 
Electroplating,  97,  147. 
Elements  and  compounds,  5. 


INDEX. 


255 


Elements  considered  molecular,  249. 

atomic  heats  of,  215. 

specific  heats  of,  212. 
Energy,  82,  172. 
Ether,  the,  83. 
Ethylene,  237. 
Eudiometer,  12. 
Eutexia,  49,  123. 
Evaporation,  54. 
Expansion,  85. 

Faraday,  M.,  29. 

Formula,  molecular,  225,  234,  236,  237,  238. 

Freezing  mixtures,  123. 

Gas,  23,  36,  57,  67,  209,  212. 
Gay-Lussac,  75,  76. 
Geissler  tubes,  24,  25,  91. 
Geology,  i. 
Gerhardt,  12. 
Germicides,  165. 
Gladstone,  J.  H.,  29. 
Gmelin,  L.,  188. 
Goniometer,  105. 
Graham,  Thos.,  29,  67,  130. 
Granite,  114. 
Gravitation,  99. 
Gunpowder,  171. 
Guthrie,  F.,  127. 

Hannay,  131. 
Heat,  42,  82,  83,  144. 

latent,  53,  54,  57. 

specific,  45,  46. 

from  chemical  combination,  184,  185. 
Heterogeneity,  17,  18,  21,  23,  25. 
History,  natural,  i. 
Hoff,  J.  H.  van  't,  118. 
Hofmann,  A.  W.,  27,  28. 
Hogarth,  131. 
Holtz  machine,  148. 
Hydrargyrum,  3. 
Hydrogen,  70. 

dioxide,  252. 

Ice  calory,  177. 
Iceland  spar,  21. 
Ice-machine,  51,  52,  53,  56. 
Infusoria,  157. 
Iodine,  183,  217,  231. 

Kekule,  32. 
Koch,  155,  156. 

Latent  heat,  53,  54,  57. 

Laurent,  12. 

Lavoisier,  A.  L.,  6,  8. 

Laws,  of  definite  proportions,  12,  72. 

of  Charles,  65. 

of  Dulong  and  Petit,  213. 

of  Gay-Lussac,  75. 

of  Henry  and  Dalton,  71. 

of  Groshans,  239. 

of  insolubility,  171. 

of  isomorphism,  104,  234. 

of  Mariotte,  63. 


Laws,  of  Mitscherlich,  234. 

of  multiple  proportions,  12,  73. 

of  maximum  work,  177. 

of  volatility,  172. 
Lead, 128. 

boro-silicate  of,  23. 
Leucomaines,  166. 
Light,  20,  22,  86,  87,  88,  146. 
Liquid,  36,  52,  59,  60,  132,  134,  239. 
Liquefaction,  47,  48,  121. 
Lockyer,  J.  N.,  29,  92,  149. 

Mallet,  J.  W.,  194. 

Mariotte,  law,  63. 

Marsh  gas,  227,  230,  232. 

Masses,  3,  6,  99. 

Matter,  3,  7,  15,  35,  38,  63,  172. 

Matthieson,  130. 

Maxwell,  Clerk,  176. 

Mechanics,  2. 

Mercury,  3. 

as  a  solvent,  119. 
Melting,  43,  46,  47,  48. 
Mendeleeff,  D.  I.,  29,  197,  243. 
Methyl  alcohol,  237. 
Meyer,  L.,  29,  244. 
Meyer,  V.,  146. 
Microbes,  150,  155,  157,  159. 
Microcrith,  3,  24,  75,  199. 
Mills,  29. 

Mitscherlich,  104,  234. 
Moistening,  115. 
Molecule,  3,  4,  15,  28,  78,  101. 
Muir,  M.  M.  P.,  176,  177. 
Mycoderma,  aceti,  158,  159. 

vini,  158. 

Nascent  state,  253. 

Naumann,  A.,  176. 

Newlands,  J.  A.  R.,  29,  197,  243. 

Newton,  Sir  Isaac,  188. 

Nitrogen,  oxides  of,  73,  74,  232,  233,  236 

Occlusion,  130. 

Organic  compounds,  151,  153. 

Organized  bodies,  105,  154. 

Osmose,  132. 

Oxygen,  study  of,  219. 

unit  of  atomic  weight,  200,  202. 
Ozone,  5,  23,  251,  252. 

Palladium,  130. 

Pasteur,  L.,  34,  155,  160,  161,  163. 

Pattinson,  128. 

Periodic  law,  243,  246. 

Perkin,  W.  H.,  241. 

Petit,  213. 

Philosophy,  natural,  2. 

Physics,  2. 

Platinum,  130. 

Polariscope,  23. 

Polarity,  102. 

Polarization,  22,  240. 

Potassium,  205,  218. 

Potassic  nitrate,  no. 

Potato  starch,  152. 


256 


INDEX. 


Pressure,  in  liquefaction  of  gases,  58. 

Protagon,  5. 

Protyle,  30. 

Front's  hypothesis,  247. 

Ptomaines,  166. 

Quartz,  112. 

Radicles,  compound,  27,  234. 
Radiometer,  40. 
Raoult,  38,  118,  241. 
Rayleigh,  194. 
Refraction,  21,  22. 
Regnault,  H.  V.,  211. 
Ruhmkorff,  148,  150. 

Saccharimeter,  23. 
Sodium,  205,  217. 

chloride,  26. 

sulphate,  126. 
Salt,  crystals  of,  in. 
Saturation,  122. 
Science,  i. 

Silver,  127,  136,  218,  223. 
Slit,  for  spectroscope,  89. 
Solid,  36,  124. 
Solubility,  of  gases,  71,  134,  135. 

of  solids,  118. 

Specific  heat,  214,  216,  247. 
Spectroscope,  86,  87,  89,  90,  91. 
Spencer,  Herbert,  29. 
Spheroidal  state,  116. 
Starch,  5,  152,  153. 
Stas,  194. 

Steam,  latent  heat  of,  53. 
Sterilizing  apparatus,  160,  162. 
Stokes,  29. 
Sugar,  no,  125. 
Sulphur,  a  study  of,  222. 

crystals,  109. 

dioxide,  56,  60,  232. 

density  of  vapor,  146,  222. 

specific  heat  of,  223. 


Sulphur  trioxide,  232. 

use  as  disinfectant,  164. 
Sulphuretted  hydrogen,  222. 
Synthesis,  26. 
Syphon,  71. 
Systems,  of  crystals,  105. 

Temperature,  58,  84. 
Theory,  atomic,  6. 
Thermal  units,  177. 
Thermo-chemistry,  183. 
Thermo-dynamics,  laws  of,  176. 
Thermometer,  65,  67,  84,  85. 
Thermolysis,  144. 
Thomson,  Sir  Wm.,  31. 
Thomsen,  Julius,  38,  176,  177. 
Type  compounds,  202,  225. 

Uranium,  200. 

Vapor,  37. 

density,  211,  213. 

Pressure,  52. 
processes,  150. 

Water,  226,  229. 

as  a  solvent,  119. 

calory,  177. 

electrolysis  of,  148. 

latent  heat  of,  47. 

spheroidal  state  of,  117. 

vapor,  77,  219. 
Weight,  atomic,  193,  196,  199,  204,  209,  245. 

molecular,  231,  239,  241. 
Wheat  starch,  152. 
Wislicenus,  34. 
Work,  191. 

Yeast  plant,  152. 

Zero,  absolute,  85. 
Zoology,  i. 
Zinc,  3,  120,  168. 


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